Warnings: Dark themes, lots of mentions of death, strong language, human experimentation, experimenting on children, child abduction, graphic descriptions of experimenting, lots of needles, lots of violence, poor descriptions of panic attacks, descriptions of blood, rated M for safety.
*If I need to add anymore warnings please tell me in the reviews or a PM and I'll be sure to add it!*
Chapter 51: The Hunt For Vander Decken
The next day after breakfast the brothers were given the ok by Marco to go exploring, so long as they come straight back if they needed any help. The brothers agreed, and just like that, the search for Vander Decken commenced.
Sabo was listening intently to their surroundings, waiting for even a hint of Vander Deckens name to be mentioned. Ace was scanning their surroundings, and seeing as he didn't know what the man looked like, opted for looking for any suspicious looking people. Luffy, much like Ace, couldn't smell right with all of this water around them, so he opted to do what he did best and scan for any bad auras that would indicate a bad person.
The three searched a lot longer then they would have preferred, the time nearing lunch before they caught a hint of anything.
They had ventured a bit closer to the edge of the bubble when Sabo heard talking off in the water, much further away from Fishman Island, located more in the depths of the ocean. It had been a brief noise, so far away that it was even hard for Sabo to hear, but he swore someone had said Decken. Seeing as this was the closest they'd gotten to any clues about the man's whereabouts Sabo shushed his brothers who had started an argument on whether it would be appropriate to eat the fish swimming outside the bubble or not.
"I think I heard someone out there talking about Decken," Sabo whispered, more focused on listening rather than the words he was saying. This shut his brothers up immediately, letting Sabo listen. They stayed silent, and Sabo could hear the murmuring in the distance. Being unable to hear the words frustrated the blonde, and after having searched this long even his patience was wearing thin.
"I can't make out what they're saying, but I know I heard someone say Decken out there," Sabo said to his brothers. Being so close to a clue, yet so far frustrated the blonde more then he would like to admit.
"Where'd you hear the talking?" Ace asked, looking around at the different fishmen and women roaming around them. Sabo shook his head, grabbing Aces shoulder and turning him to look outside the bubble they were in.
"It wasn't here on the island, it was out there." He said, pointing off into the distance. Ace and Luffy frowned at that, the youngest squinting in an attempt to see something while Ace used his enhanced vision to take a look.
"Oh. I don't see anything out there or anyone. It's just a bunch of fish... some coral... a giant- oh, I see a ship!" Ace said with a grin, excitement rising in him at the prospect of actually finding the man they were most likely going to fight.
"Then let's go!" Luffy said happily, nearly charging right at the bubble. Before he had a chance to though, Sabo stopped him.
"Now hold on, how exactly are we going to get over there? Not only do we have to somehow get through this bubble, but we also have to get through the next bubble surrounding this one. Also, did you two fail to remember that that's water out there and you're both devil fruit users? And not to mention we're hundreds, maybe thousands, of feet under the water? I'm not a superhuman, that kind of pressure would kill me." Sabo said, arms folded over his chest as he frowned at his over-eager brothers. Ace frowned in annoyance, mimicking Sabos stance and folding his arms as well.
"What are we supposed to do then? I'm not just gonna let the crybaby princess stay in that room the rest of her life because of some stupid water." Ace said, Luffy nodding his agreeance.
"Yeah, me neither!" Luffy shouted. This caused Sabo to sigh, knowing just as well as his brothers that none of them wanted to let that happen.
"And you think I do? This just means we have to figure out how we're going to get out there without drowning. Not only that, but we also have to figure out how to get out there in the first place." Sabo said, eyeing the protective bubble around them.
"Why don't we just walk through it?" Luffy asked, Ace nodding as he obviously thought the same thing. Sabo took a deep breath, trying to stay patient, knowing that sometimes his brothers weren't the brightest.
"The bubbles are tough, remember? And that was just a bubble around the ship, I have no doubt that this giant bubble surrounding the island would be much tougher and harder to get through. Not only that but there's another bubble around this one, meaning that even if we managed to get out of this one, we'd still have to get out the other." Sabo explained, pushing his hand on the bubble to prove his point that it wouldn't go through so easy.
"So we just need something to push us out, right?" Ace asked, and Sabo couldn't stop the slightly frustrated sigh that left his lips then.
"Ace, were you not listening? We'd need a lot of force behind us to get us through this." Sabo said, gesturing once more to the giant bubble. Ace pouted and crossed his arms, also becoming annoyed.
"Of course I was listening! I was just thinking, what about Luffys rocket? Couldn't that get us through?" Ace asked, and suddenly the frustration and impatience left Sabo as he felt slightly guilty about getting mad at his brother when he was only trying to help.
"Oh, well, that might work, but it would probably only get us through one of the layers if we were lucky, and then we'd still be stuck in the second layer where Luffy wouldn't have anything to grab onto to rocket us. It was a good thought though." Sabo said, giving his brother a slightly apologetic smile. It went silent again as they thought about their options, and soon it became obvious to them that they didn't have many.
"We can't use the entrance, I doubt the guards would just let us walk out there, even if we are Whitebeards, we're still kids. Not to mention, it would be weird if we left Fishman Island and came back with Vander Decken." Sabo said, thinking out loud as he began pacing back and forth, trying to think up a solution.
"What about the guys that're leaving?" Luffy asked, looking over at something. Confused, his brothers looked as well and saw that there was a small group of fishmen loading up a relatively small boat, looking to be heading out.
"Well... that could work, but what about the whole drowning thing?" Sabo asked, watching the men carefully.
"Bubbly coral!" Luffy provided happily, holding up their bubbly coral proudly, and Sabo felt a bit dumb for having forgotten about the wonderful piece of technology.
"Why can't you be this smart all the time?"Ace asked, a smile on his face as he took the coral from his brother. Luffy, expectedly, pouted at the insult, but Sabo didn't give them time to argue.
"If we're doing this then we have to go now, the boats about ready to leave," Sabo said, hearing some fishmen calling that everything was loaded and they were ready to go. Faces serious now, Ace and Luffy both nodded and the trio headed to the ship, glancing around to make sure no one was watching them too closely. It was relatively easy to sneak onto the boat, all they had to do was silently follow the last guy up the gangplank and disappear from sight before anyone saw them. Apparently, the crew wasn't worried about stowaways because no one looked for them and no one expected them to be there.
It was all too simple to slip off to the many stacks of boxes and barrels and hide behind them. It just so happened that their hiding spit was right beside the railing as well, giving them a perfect escape route. The ship began to disembark so Sabo took the opportunity to use the bubbly coral to surround him and his brothers in a bubble. With that done, all they had to do was wait and hope no one managed to stumble upon them.
The ride out was quick and easy, the ship leaving through the entrance they had entered with the day before, and before they knew it they were jumping off the boat and out of the protective coating around it, leaving them alone in the depths of the ocean.
"Alright Ace, nows a good time to use that supervision of yours and guide us to Vander Decken," Sabo said as he looked around blindly for the ship Ace had seen, but having no luck in finding it. Ace nodded and glanced around for it, promptly finding it somewhere over to their left.
"It's over there, a bit away, but it shouldn't take too long to get to it. Come on." Ace said, and the three began to make their way over to the ship, slowly but steadily.
"Can you see anyone on the ship?" Sabo asked, wanting to know what they were up against. Being out in the ocean now everything sounded a bit muffled to him. He could still hear things, like the people on the ship, but he couldn't make out what they were saying as easily as normal or figure out how many different voices there were.
"Yeah, a pretty good bunch of people. I think its a pirate ship. Hey, is this Decken guy a pirate or something?" Ace asked, noticing the skull waving proudly on the black flag.
"I don't know, Princess Shirahoshi never mentioned it," Sabo said with a shrug, not caring if this man was a pirate or a king.
"We get to beat up some pirates? Cool!" Luffy cheered happily, arms stretching out above him and in turn stretching the bubble.
"Why don't we be a bit more careful Lu? I'd rather not test this bubbles limits while we're depending on it to keep us all from dying," Sabo said, sweatdropping as he gently lowered Luffy's arms.
"Isn't it supposed to be pretty tough? Besides, we're about to find out soon seeing as we have to fight these guys and they don't look like they're using a bubble around their ship." Ace said, squinting to get a better look. Sabo sighed in defeat, knowing his brother was right and hoping that the bubble would hold up. Really, they should have planned this better, or at least have tested the bubble to see how tough it was, but it was too late to turn back now and there was no way they were going back empty-handed.
Soon enough, the brothers reached the ship, the Flying Dutchman according to its sails, and it was easy to just swim right onto the deck seeing as there was nothing stopping them.
'Try haki.' Sabo tapped quickly before anyone noticed they were there. Ace nodded, and while he blasted the fishmen in front of them Sabo used the bubbly coral to give the brothers their own separate bubbles, allowing them to move more freely.
The haki clearly affected most of the pirates, a lot of them falling to the ground foaming at the mouth. Meanwhile, a few kneeled at the pressure and one remained standing, only stumbling a bit. When they had reached the ship Sabo had been able to make out the talking much clearer, and if he had to guess, the one standing was Decken according to the many, now fallen, pirates. He also seemed to be the Captain of this crew as well. Though the man was definitely a Fishman, he had a bubble around him as well, confusing the brothers. Couldn't fishmen breathe under water? Why would he have a bubble around him if he could supposedly breathe just fine like his crew around them?
And then Sabo remembered the fact that Vander Decken supposedly had a devil fruit. It was strange that the sea would affect even a Fishman, but Sabo knew not to question the mysterious powers of the fruit.
"Oh? And who are you?" Vander Decken asked, eyeing the three brothers with a raised eyebrow and a frown. Seeing as most of his crew was down on the floor and they hadn't even touched anyone yet meant that one of them had the third rare haki, something Decken knew to be wary of. Even so, they were just kids.
"We're here to ask you to stop sending Princess Shirahoshi weapons to her door," Sabo said with a polite smile, knowing full well the man wasn't going to stop just because they asked.
"So Shirahoshi sent you then, hm? No matter, I will have the princess marry me! Her beauty is meant only for me and if she can't accept that then she will just have to die." Decken announced, his crewmates nodding their agreement and cheering their Captain on, all getting ready to fight.
"As you can see, I won't be stopping anytime soon, and there's nothing three kids can do to make me change my mind." The Captain said smugly, crossing his arms over his chest. All of the brothers smiled now, Ace and Luffy cracking their knuckles in anticipation while Sabo tilted his head.
"Actually, I think there is." The blonde said matter of factly. At that moment one of the fishmen seemed to notice the symbol on their wristbands, their eyes going wide.
"C-Captain, they're with the W-Whitebeards!" The Fishman stuttered, pointing to the emblem. The entire crews' eyes went wide, even Deckens.
"What?! But they're just a bunch of kids!" Decken shouted in surprise, the brothers' smiles never faltering for a second.
"No matter, we can take them on and let their bodies sink to the sea floor! That old man will never know what happened to 'em! Hear that? Let's get 'em!" Vander Decken shouted, pointing to the brothers. The few remaining crew members cheered and rushed forward toward the three.
"Luffy and I can handle them Ace, you go get Decken," Sabo said, knowing the eldest would be best suited to fight the captain. He was always the strongest out of them, though he would never admit that out loud to him.
Aces grin turned malicious as he nodded, fists formed as he rushed toward the Captain, his brothers following to stop anyone who would get in the way.
"Aw, but I wanted to fight him!" Luffy whined as he easily punched a man that had tried to swing their sword at him.
"I'm older, so that means I get the bigger fish!" Ace shouted, sticking his tongue out at Luffy as he ran past the crew and towards Decken, leaving his brothers to fight.
"No fair." Luffy pouted, knocking another two fishmen out with his whip, flinging them into the side of the cabin.
"If we hurry up here we can join Ace in beating him up," Sabo said with a smile as he knocked the many legs out from under an octopus Fishman, the man falling to the ground where Sabo stomped on his chest. The man wheezed in pain and Sabo kicked him to the side as he took on another two fishmen who tried converging on him. Jumping into the air -water?- he dodged the swipes of their swords and kicked the back of one of them, sending him flying into his crewmate and launching them into the same man Luffy had kicked into the cabin wall.
"Yosh! I'm gonna beat 'em all up!" Luffy cheered, knocking two heads together. Sabo and Luffy were doing fairly well, watching each others backs so neither got a cut or scratch on them. Within minutes they were done and they felt nice and warmed up. It had been a while since they had been able to fight, and it felt nice to get the exercise, especially for Luffy. He hadn't been able to stretch his limbs that far in a long time and it felt amazing. It was a good thing that none of the fishmen had a chance to notice the boys' devil fruit before they were knocked out.
Turning to look at how Ace was fairing they saw that it was an easily one-sided fight with Ace being the obvious victor. Decken was covered in bruises and his face was already beginning to swell from how hard Ace had punched him. Sabo couldn't help but chuckle at how overboard Ace had gone while Luffy was pouting again, mumbling that he wanted to fight 'Dellen' too.
"If you wanted to fight him you should've beaten the others quicker." Ace said, having heard Luffy's complaints. There wasn't a single scratch on Ace either, and he wasn't even out of breath from the fight.
"I thought these guys were supposed to be strong," Sabo said, nudging Deckens motionless hand with his foot in slight disappointment. Either the brothers were much stronger then they had thought, or these guys were much weaker than average.
"Wasn't that strong to me." Ace said, slightly annoyed that the Captain hadn't put up much of a fight. In the middle of the man boasting about how strong he was and how weak Ace was Ace had sent a fiery fist at the man's face and knocked him out instantly. All the other punches were Ace venting his pent up frustrations.
"So what now?" The eldest asked, crossing his arms and looking to the blonde.
"Well, we bring him back." He said with a shrug, lifting the unconscious Captain and putting him on his back, getting ready to make the swim back to Fishman Island.
"Just like that?" Ace asked with a raised eyebrow.
"Well, yeah, there's not much else we can do," Sabo said as he walked to the railing of the ship, his brothers following.
"But I thought we had to hide how strong we were," Luffy said in confusion, tilting his head. Sabo nodded jumping off the railing and beginning the swim back.
"You're right, we do, that's why we're going to fib about how we defeated Decken. Ace, lead us back to the entrance." Sabo said, and Ace mindlessly got to the front and began leading the way.
"How are we going to explain the very obvious wounds?" Ace asked, his eyebrow raised.
"Well, this definitely would have been easier if you hadn't made it so obvious that someone attacked him, but I expected you to do something like this so I already had a story ready. Don't worry, it'll be fine." Sabo said, moving Decken a bit to make it easier to swim while carrying him. They didn't speak after that, Ace and Luffy trusting that Sabo would handle things as they came. It took a bit longer to get to the entrance seeing as they were just swimming back, but eventually, they made it, and as Sabo anticipated the voice of the guards inside boomed out to them through speakers.
"What are you three doing out there?" They asked, clearly baffled by the three teens making their way to the Island.
"Well, we found something that you might want to come take care of," Sabo said, nudging his shoulder that Deckens head was resting on, giving them a view of the fishmans beaten appearance.
"Is that-? Oi, call the royal palace! Tell them some kids got Vander Decken!" The noise coming from the speakers suddenly became frantic as there was chaos inside, all the guards moving as fast as they could to get the message to the palace.
"You three just stay there, the Royal Highness will be by soon." The voice said eventually to them, and Sabo nodded, the brothers getting comfy as they waited.
"So far so good." Ace said arms crossed as he looked at the giant entrance. Sabo nodded.
"Yeah, let's just hope it stays that way," Sabo said. Sure, he had a plan, an explanation for all of this, but even so, it was a bit far fetched. However, there would be no other way of explaining how they managed to take down the infamous Vander Decken without telling them they had attacked and defeated him and his entire crew.
"I think everything's going to be ok! Sabo's smart!" Luffy said in reassurance, most likely having picked up on all the nervousness coming from both Ace and Sabo. The two oldest smiled at him and nodded.
"Yeah, you're right Lu, there's nothing to worry about." Ace agreed, nodding his head. Luffy felt the nervousness seemingly melt away from his brothers as they calmed down, Luffy's words having helped them immensely. It was always strange to the two older brothers how Luffy could seemingly calm their emotions if he tried, no matter how anxious or scared they were.
Minutes ticked by slowly, and after about 20 minutes had passed the guard announced that Neptune was there. The man was on his giant whale, arms crossed and face serious as they came out into the ocean with the brothers.
"You got Vander Decken, jamon?" He asked skeptically.
"We did your highness," Sabo said, swimming up to the King with his brothers and handing the defeated Captain over. Neptunes' eyes widened in surprise, clearly not having expected it to be true, but now that he was seeing it, he was speechless.
"I-I... I don't know what to say..." He trailed off, staring at the unconscious man sitting in front of him.
"Thank you. You've done a great service to this Island! Come, we must get back to the Palace at once! I'll send for the Whitebeards to join us, jamon!" Neptune said, his joy and relief exuding from him in waves, making Luffy fill with joy and relief in response.
The brothers were ushered onto the whale and then were brought back into Fishman Island where they headed straight for Ryugu Palace. As they got close Sabo could hear the guards inside whispering about rumors they'd heard that Vander Decken had been defeated, but the second they actually reached the palace and went inside, everyone grew silent, standing tall and faces void of emotion.
The brothers and Neptune got off of the whale, Neptune holding Vander Decken, and the trio followed the King as he walked up to the ministers and a few guards. Tossing Vander Decken over to the guards he gave them a serious look.
"Lock him up, jamon and make sure there's no way he can get out." He said to the guards, who saluted and grabbed the man, quickly rushing to the prison cells.
"So the rumors are true then." Minister of the Right said as he watched Vander Decken disappear from sight. Neptune nodded, a smile blossoming once more.
"They are. Thanks to our friends here, Shirahoshi should finally be safe from Deckens advances, jamon!" Neptune cheered happily, the ministers smiling as well in relief.
"Then you have our eternal gratitude." Minister of the Left said, bowing to the brothers who didn't quite know how to react to the attention, especially since the Whitebeards weren't there yet.
"Ah, you know, it was nothing," Sabo said, rubbing the back of his head sheepishly. At that moment there was a ringing from a snail phone, which one of the ministers promptly moved to, answering it. It was the princes, who the brothers had yet to meet, saying they had arrived with the Whitebeards. The Minister opened the channel for them to get in and the group waited in silence for the pirates to arrive.
"Oh, Ace, Sabo, Luffy, you're here too?" Thatch asked as he, Marco, Haruta, and Namur walked in, the three Princes trailing in behind them. The brothers only waved to the pirates in response as Marco looked up at Neptune.
"What was the purpose of calling us all here, yoi?" He asked. Luffy could feel that he was a bit worried but mostly curious and Luffy could tell the worry was directed toward the brothers. Did he think they were in trouble?
"Why, for a celebration, of course, jamon!" Neptune said cheerily, smile wide on his face.
"A celebration? For what?" Haruta asked, also confused, along with the other Commanders.
"Well, it would seem you're new recruits are quite amazing indeed. They managed to find and defeat Vander Decken, Jamon." Neptune said, the smile never leaving his face. The Commanders all became wide-eyed, turning to look at the brothers in astonishment.
"You defeated Vander Decken?" Namur asked. They had all heard about the man, he was the reason for keeping the princess locked away all this time, but they hadn't been able to find him anywhere on the island whenever they were around.
"Well, not exactly," Sabo said, beginning the lie.
"I'm sure they can recount the story at the feast." Minister of the Right said, ushering the group into the dining hall they had been in previously where there was once again a feast being held. Luffys mouth watered at the memory of how good yesterdays feast had been, and he and Ace immediately rushed to sit down, Sabo following at a much more average pace.
As food was passed around and everyone became settled, all eyes were on the brothers, everyone wanting to know just what had happened. To be honest, Ace and Luffy were curious as well, their eyes turning to Sabo as they waited for him to begin explaining.
"I guess I'll start from the beginning. My brothers and I were walking around Fishman Island and we got close to the docks where we saw someone outside the bubbles who looked to be drowning. We asked one of the leaving merchants to take us out there and used the bubbly coral to swim over and saw that it was Vander Decken, which confused us because he's a Fishman." Sabo began, and he could already see that everyone was listening with rapt attention, the Whitebeards especially.
"So we pulled him into our bubble and he started muttering random things like Shirahoshi and how he was going to marry her, and he said his own name too which is how we knew who he was. When we got lost here yesterday we heard some people talking about the mermaid princess and how she was confined to her room because of Decken, so we knew he was a bad guy, and before he regained his senses Ace started hitting him... a couple of times." Sabo said with a shrug as if it wasn't a big deal.
"After he seemed to be incapacitated we brought him back to the entrance where the guards called King Neptune and they imprisoned him." Sabo finished, eating calmly to keep up the rouse that their story was true. Everyone seemed to buy the story, and a check with Luffy proved that no one was suspicious of them.
"Magnificent! Thank you for the service you've done to the Kingdom, it can never be repaid, Jamon." Neptune said, his smile never leaving his face the entire time.
"Really, it was no problem, we didn't even do that much," Sabo said, holding his hands up as he shook his head.
"It may not seem that way, but the entire kingdom has been looking for Decken for years. To think, he was outside the bubble this entire time." Fukaboshi spoke up finally, his brothers nodding their agreement beside him.
"Yes, why don't you and your brothers go get Shirahoshi. I'm sure she would like to finally come out of her room and meet her saviors, jamon." Neptune said to the princes who nodded and immediately headed toward the tower Shirahoshi was in. Everyone began finishing up their meals and began heading back to the main hall of the palace.
'You don't think that crybaby princess is going to give us away, do you?' Ace asked as they followed Marco and Thatch closely.
'I don't think so, after all, we did help her as we promised.' Sabo said, also a bit nervous that she would, whether on purpose or not.
'I don't think she will, Shibahoshi's a good person!' Luffy replied confidently.
'Yeah, that's true. Also, its Shirahoshi you two. Especially here in the palace you have to call her by her actual name, not crybaby, and not Shibahoshi, got it?' Sabo asked sternly, knowing his brothers were stubborn about things like that.
'Yeah, yeah, whatever.' Ace replied, and Sabo just had to hope the eldest would listen. Even though Sabo had just warned Ace about his name calling, he could already hear said Princess heading their way, tears and sobs already making their way to his enhanced senses.
"Ah, Shirahoshi!" Neptune greeted as the giant mermaid entered the hall, her brothers following close behind.
"Father, is it true?" Shirahoshi asked, looking towards the brothers for only a second before looking back to the king. Her tears, as expected, were falling fast and hard.
"It is, these three brought Vander Decken here themselves, jamon," Neptune said, gesturing to the brothers. Sabo and Luffy were smiling to the princess while Ace looked as if he was uninterested, though he was a bit antsy at all the sudden attention they were now getting.
"Really? Thank you!" Shirahoshi thanked loudly, bowing her head to the brothers gratefully.
"That's not necessary, we didn't do that much princess. Its a pleasure to meet you, I'm Sabo, and these are my brothers, Ace and Luffy." Sabo said, pointing out his brothers to the princess, regardless of the fact that she already knew them, and bowed, making sure Ace and Luffy did the same.
"It's nice to meet you as well, I'm Princess Shirahoshi!" She returned, a smile widening happily on her face, even as her tears continued to fall rapidly.
"Shirahoshi, my daughter, thanks to these three, from this day forth, you can finally leave your room freely without worry, jamon," Neptune said happily, tears of his own forming in his eyes, though they didn't fall. Shirahoshi didn't respond, instead, she wiped her tears, which didn't do much to help, and nodded.
/\/\/\/\/\
"I'm sleepy!" Luffy said as he yawned, stretching his arms above him, though controlling how far they went. The group of Whitebeards and honorary Whitebeards were heading back to the ship for the night, the sky nearly completely dark now.
"Don't worry Lu, we can go to bed once we're back at the ship," Sabo said, patting Luffys back.
"I still can't believe you three managed to mangle Deckens face up that much." Thatch said, still amazed. The Whitebeards had gone and visited Vander Decken before they left to make sure the guards didn't have trouble locking him up, but mostly because they wanted to see with their own eyes that their defenseless, traumatized trio had managed to take down a man that had evaded them for years.
"Ah, well, after we heard from the guards about how Shirahoshi had been kept in her room for years we weren't exactly big fans of the guy. When we figured out who it was Ace went a little overboard, and I wasn't going to stop him so I let him." Sabo said with a shrug. Ace crossed his arms, frowning a bit as he remembered how easy it was to take the man down. He really had been hoping for a more entertaining fight.
"He was a bastard." Ace said matter of factly, Luffy nodding his agreement. The Whitebeards watched the brothers for a second, suddenly remembering that, though not quite in the same way, they had been locked up for years against their will. All of a sudden, they wished they had been able to punch the guy a few times.
"I'm glad that everything went well in the end, yoi," Marco said, face giving no indication on his emotions.
Sabo was just thankful that he hadn't been mad about them leaving the island.
Decken has been captured! The brothers, thanks to their enhanced senses, were able to do it easily. I got a question last chapter on whether Luffy should be able to track Decken due to the scent on the sword he threw, however, as I briefly mentioned in the very beginning of this chapter, they're surrounded by water which is hindering Luffys ability to smell anything outside of the bubble, hence why he couldn't smell Decken. It was a good thought though, and if they were anywhere other than underwater, he definitely would have been able to pick up the smell and track it.
And now, about the slightly anti climactic fight. As mentioned throughout this story so far, the brothers are much more proficient at fighting then they had previously been at this point in canon. However, I've also stated that they won't be overpowered. In my mind, everything at the facility gave them a jump start on their training to the point that they're as strong as they were once Luffy started his adventure at 17. So, instead of Luffy being as weak as he was at 14, he's now as strong as when he could take down Don Kreig and Enel on his own, but unable to damage someone like Kizaru. Its the same for Ace and Sabo, except they're as strong as they were at 20, where Ace could easily distract Smoker but have difficulty against Akainu. Vander Decken is a New World villain, but the brothers are all together and, therefore, stronger than when they're by themselves which allowed them to take him down rather quickly. As it's been seen in canon, they were always stronger when they fought together. Finally, they've all had a bit of training in haki which also helps them out.
Also, in canon, Luffy literally beat the fishman in one hit with his hands tied behind his back, so its safe to say he wasn't really all that strong.
I hope you all enjoyed the chapter! I'll see you all next week with the next one!