Even with the information Dumbledore had left for them, it still took Sirius and Remus longer to find the cave they thought Voldemort had hidden a Horcrux in than what they'd initially estimated. It was only once they visited the area, that they'd realised the scale of their search. The cave was located on a rugged muggle coastline, and there had been dozens of potential caves Voldemort could have used to hide his Horcrux. However, with a bit of searching Remus and Lupin located what they thought was the correct cave in the first week of November.
"This is definitely it," Remus said from where he and Sirius were perched on a ledge overlooking the sea.
"Really?" Sirius looked around sceptically. "We've gotten here by magic, but how the hell did You-Know-Who get here as a child. I thought Dumbledore picked this place because of and incident in his childhood."
"Things could have been different back then," Remus suggested with a shrug. "The cave might not have been so hard to reach."
"You would hope so, otherwise a child would have no chance of reaching it," Sirius muttered.
"Does it really matter right now?" Remus questioned. "What matters is getting inside the cave and searching it."
"And that's not going to be easy," Sirius remarked, turning his attention to the cave entrance.
A large boulder covered the entrance, and there was a chunk of the ledge they were standing on missing, leading the two wizards to suspect that there'd been a cave in at some time. Anywhere else and they might have just tried to blast their way into the cave, but they were wary of going in hard. One wrong move could bring the entire cave down on them, not to mention the surrounding cliffs. Neither Remus nor Sirius wanted to be responsible for what could potentially be a devastating accident in the muggle world.
"I think what we need is patience and thought," Remus said, giving his best friend a warning glance.
"I can be patient," Sirius protested.
Remus didn't reply to his friend, he just chuckled quietly and set about examining the large boulder blocking the entrance for any way through. Muttering under his breath about how he was perfectly capable of being sensible, Sirius joined his friend in examining their first hurdle.
"This boulder hasn't fallen into place, it's been placed here," Sirius announced after a few minutes examination. "It's meant to look like a cave in, but it's not."
"Are you sure?" Remus asked hesitantly.
"I'm sure," Sirius replied with a confident not. "I think this cave is stronger than we thought."
"You want to use force," Remus stated with a sigh.
"Worth a shot," Sirius replied with a grin as he pulled out his wand and backed up as far as he could on the ledge. Once Remus had also backed away from the ledge, he raised his wand and focusing on the base of the rock, he shot off a spell. "Bombarda!"
Sirius's spell flew into the boulder, and a few small rocks splintered off, but the main boulder held firm. Throwing a few more blasting spells at the boulder, Sirius only succeeded in creating some smaller rocks from the boulder.
"Maybe we should try it together," Remus suggested as he got his wand out.
"Okay," Sirius replied with a nod. "One the count of three. One, two, three. Bombarda!"
"Bombarda!" Remus called at the same time as his friend and two bolts of magic flew towards the boulder.
The two spells hit the boulder at the same instant, and they dislodged yet another small bit of rock. However, the rock came flying back towards the pair and hit Sirius squarely in the forehead.
"Bugger," Sirius spat, falling to his knees with the force of the impact.
"Are you okay?" Remus checked.
"Fine," Sirius muttered, swiping his hand across his head and finding the rock had broken the skin and he was now bleeding.
Ignoring the bit of blood, Sirius surged to his feet and stormed up to the boulder. When he found that all they'd managed to do with their combined magic was create a small crack in the surface, he slammed his bloody hand into the boulder with a cry of annoyance. At this rate they would be at the boulder all day, just trying to blast their way into the cave.
"Careful," Remus warned his friend. However, before he got a chance to pull Sirius away from the boulder to check out his injury there was a creaking noise and the boulder began to slide to one side.
"What the hell?" Sirius muttered, backing away in shock.
"Of course," Remus tutted, mentally slapping himself for not being on the ball. "Blood magic. A blood sacrifice opens the cave."
"A bit primitive isn't it?" Sirius snorted.
"A lot of dark magic incorporates blood magic," Remus replied. "We need to remember that as we advance. We need to think dark."
"Right now that isn't a problem," Sirius retorted as he eyed the entrance to the cave.
"Not so fast," Remus called, stopping his friend before they went any further. "Let's deal with the cut on your head first."
Sirius protested, claiming he was just fine, but Remus was adamant that he got some medical treatment before they moved on. Luckily, it was just a graze and it didn't take long before the two wizards were ready to venture into the cave. Lighting their wands, they stepped into the cave, ready to face whatever Voldemort had in store for them.
What followed was a two hour trek through a dark cave, where they had to overcome several blockages of dark magic. However, it wasn't a constant stream of magic they had to face, more often than not just negating the cave itself was enough of an obstacle. After two hours both Sirius and Lupin were sure they were deep in the bowels of the cliffs. Escape was not going to be easy, but at least they didn't have to pass through all the dark magic again as they'd dealt with it all on the way down.
"Is this it?" Sirius asked as they reached the edge of what looked like an underground lake.
"I think there's an island in the middle of the lake," Remus replied, holding up his lighted wand so they could get a better view.
"Now we just have to get over there," Sirius said, looking from side to side for a bridge of some sort. He didn't really fancy swimming over to the island, but he would if he had to.
Remus was also looking around, and he spotted a rope disappearing into the water. Picking up the rope, he began to pull on it and as he did so, a small boat began to rise from the water.
"Excellent," Sirius beamed, advancing to the edge of the water to help pull the boat closer.
As Sirius helped Remus with the boat, he got a bit too close to the water and his foot ended up splashing into the lake. The second his foot was in the lake, something grabbed hold of it and Sirius found himself pulled to the ground as whatever had hold of him tried to pull him into the water.
"Remus!"
"Sirius." Remus was at his friend's side in an instant and pulled Sirius out of the grasp of whatever had hold of him.
However, in doing so, he'd also ended up with his foot in the water and whatever had hold of Sirius had turned it's attention to him. Or at least, that was how it initially appeared, but the second the mysterious creature in the water got hold of him, it released him. In fact the creature didn't just release Remus, it flung him right out of the water.
"What was that thing?" Sirius panted as he looked down at his leg and found his trouser leg had been shredded by whatever had hold of him.
"I don't know, but it doesn't like me," Remus mused. "It might be my werewolf blood. Hopefully it'll leave us alone now."
"Hopefully," Sirius agreed as he got back to his feet and turned his attention back to the boat they'd been trying to reach.
The boat had vanished, but the rope was still visible, so making sure not to get too close to the edge of the water, the two wizards hauled the small boat to the surface once again. Not wanting to mess around, the pair climbed into the boat as soon as it was near to shore, and using magic they propelled it across to the island. As they travelled, they were aware of creatures floating in the water alongside the boat, but possibly thanks to Remus's werewolf heritage, the mysterious creatures remained in the water.
When the boat docked the pair hopped out as quickly as they could and they scoured the small island. However, the only thing on the island was an ornate font, filled with water, and a silver chalice sitting on the rim of the font.
"Is it the cup, do you think?" Remus questioned, picking up the chalice as Sirius peered into the font.
"I don't think so," Sirius replied. "Come and have a look."
When Remus peered into the font, he spotted a locket sitting at the bottom of the water. However, when Sirius went to reach into the font to grab the locket he found his fingers running over stone. Either the locket was an illusion, or it was hidden in some way which made it impossible to just grab it from the font.
"I think we need to remove the contents of the font," Remus said.
"Worth a shot," Sirius agreed. Grabbing the chalice off Remus, he scooped out a glass full of water and threw it on the floor. However, once the water hit the floor, the font seemed to refill itself.
"I don't think that's the way," Remus said with a sigh. "To get to the locket, someone has to drink the water, and I'm guessing it's poisoned."
"Whoever drinks it will likely die," Sirius pointed out. "It's going to take us another couple of hours to get back out of the cave. That might not be enough time."
"It's a chance we're going to have to take," Remus replied. "I know for a fact we've got a bezoar back at your place, so whatever the poison is we have a cure."
"Providing whoever drinks the poison lives that long," Sirius muttered.
"I'll take the chance," Remus said, taking the chalice from his friend's hand.
"Why you?" Sirius demanded, snatching the chalice back off Remus.
"The werewolf genes in me will fight whatever poison this is, hopefully giving us enough time to get back home," Remus explained. "You know it takes a lot to fell me."
Despite the fact he didn't want his friend endangering himself, Sirius had to agree it made more sense for Remus to take the chance with the potion. Being a werewolf didn't exactly make him immune to poisons, but it took longer for them to work their way through his system.
With the decision made, Remus grabbed himself a chalice full of liquid. Quickly drowning it, he refilled the chalice and poured that down his neck as well. As he drank they both waited for whatever affect the liquid had to take hold of Remus, but nothing happened. The only thing that happened was that the font slowly began to empty. With every glass Remus drank, Sirius tried to get the locket, but it wasn't until the font was practically bone dry that he was able to grab it.
"Got it," He declared triumphantly.
"Good," Remus replied in a slurred voice.
"Remus." Sirius was at his friend's side in an instant as Remus began to sway on the spot. "Hold on, I'll get you out of here."
Tucking the locket safely into his pocket, Sirius helped Remus back into the boat and he steered it back to shore. The trip was slightly rougher than the first one as the creatures in the water seemed to sense that Remus wasn't a threat at the moment. However, Sirius was able to get the boat back to the shore without any major incident. Although, getting out of the boat wasn't easy as Remus could barely stand and Sirius had to practically drag his friend away from the water and whatever creatures lurked in their depths.
"Don't worry Remus, I'll get you out of here," Sirius vowed.
Even though it was a struggle, Sirius managed to get Remus back through the cave. The second they were out of the confines of the cave, and it was safe to apparate, Sirius pulled put his wand and rushed them both back to Grimmauld Place. Both Molly and Tonks were at headquarters, and they immediately rushed to Sirius's aid as he arrived with a nearly unconscious Remus.
"What happened?" Tonks asked in horror when she laid eyes on Remus.
"Some sort of poison," Sirius answered. "Remus mentioned a bezoar, we need it now."
Molly rushed off and got the bezoar, and once Remus had swallowed it, he slowly started to improve. However, he was still settled in bed and Sirius's focus was on firmly on his best friend. As he made sure his best friend was alright, Sirius didn't give the locket in his pocket another thought. In fact it was several hours later, once he was completely sure Remus was going to be alright, before Sirius pulled the locket from his pocket to examine it.
As Sirius pulled the locket from his pocket, his thumb caught a catch on the side and the locket opened up, causing a small sheet of parchment to flutter to the ground. Picking up the note, Sirius frowned when he recognised the scrawl on the paper as belonging to his brother. Reading the paper, he found it was a note for Voldemort, explaining that the Horcrux had been taken and replaced with a fake.
"Regulus what did you do?" Sirius whispered, wondering if the Horcrux had anything to do with his brother's death.
His brother had joined the Death Eaters willingly, but when he'd tried to back off and leave them, he'd been killed. At least that was what Sirius had always believed, but with this note he now wasn't so sure. Maybe he'd gotten killed for some other reason. Although if he had been killed for stealing the Horcrux, why had Voldemort left the fake Horcrux in it's hiding place? Was it to fool the likes of Dumbledore if he ever stumbled on his secret, or he was he unaware that his locket had been replaced? Whatever the answer was, Sirius was sure he wasn't going to find out the truth all these years later. All he could do was hope that Regulus had hid the locket before Voldemort killed him, and in that case there was a very good chance that it was hidden somewhere in Grimmauld Place. All Sirius had to do now, was find it.