AN: Hello! Here again with another 'Tensei Shitara Slime Datta Ken' story.
This time, our beloved slime will meet the first Avatar that was introduced to us, the last airbender participating in his first crossover.
The Legend of Aang is set after 'The Quest' with slight changes.
Tensei Shitara Slime Datta Ken is set after the second season.
I don't own anything.
The sun was shining on Rimuru City which had recovered after the recent events concerning the kingdom of Falmuth.
However, every cloud had a silver lining, for thanks to that event, Rimuru Tempest, a human who upon his death was reincarnated as a slime and befriended by the mighty Storm Dragon Veldora, rose in rank and acquired the title of Demon Lord.
Not only that, but during the Walpurgis, the meeting of the Demon Lord, he was able to take revenge on Clayman, the cause of it all, and after that, he officially became part of Octagram, the group of Demon Lord to which he gave its name.
Now Rimuru was, in his human form, in his office, signing some papers, for being the ruler of the Jura Tempest Federation, he couldn't take many days off unless they were for extreme necessity, like when he had to be absent to help Shizue's students.
"And with that I'm done." said Rimuru after filling out a piece of paper that he placed on a stack before letting out a sigh of relief.
That's when the door opened and Shion entered the room while carrying a stack of papers.
"Lord Rimuru. Here I bring you the latest reports." said Shion.
Rimuru let out a sigh, but this time, of exhaustion.
"Understood. I'll take care of it." said Rimuru as Shion put the papers down on the desk.
"You look tired, Lord Rimuru. How about I fix you something to lift your spirits?" asked Shion with a big smile.
"N-No need, Shion. I'm fine." said Rimuru with a nervous smile.
"Oh, it's okay. Call me if you need me." said Shion walking out of the office.
"Her food may taste better thanks to her 'Master Chef' skill, but the appearance still leaves a lot to be desired." thought Rimuru.
"Analysis. You're a coward." said Raphael.
"Yeah, but I'd rather that than break her heart with the truth." thought Rimuru.
He was about to get down to the papers when suddenly, a light appeared in the office and as it brightened, it revealed someone the human slime knew.
"Treyni?" asked Rimuru surprised to see the dryad.
"I hope I'm not interrupting something important, Lord Rimuru." said Treyni.
"Don't worry. Tell me, what is it?" asked Rimuru with a smile.
"You see, Lord Rimuru. I know this may sound strange, but a friend of mine needs your help." said Treyni.
"My help? Of course. Who is your friend? Another dryad?" asked Rimuru.
"No. The truth is… he's not even from this world." said Treyni.
"Huh? Can you explain?" asked Rimuru confused.
"There is a world apart from the physical world known as the spirit world. In that world, spirits can live in peace away from the beings of the physical world, though there are few who can enter there." said Treyni.
"And since you are the spirit guardian of the forest, you can visit that world, can't you?" asked Rimuru.
"That's right. I made several friends there and one of them comes from a world very different from this one and apparently that world is in trouble and I told him about you and your abilities." said Treyni.
"I understand. And your friend thinks I can help him with his world's problem." said Rimuru.
"Exactly. I know it may sound selfish of me, but I'd like you to help him." said Treyni.
"It's not selfish at all. If your friend needs help, I will gladly give it to him." said Rimuru with a smile.
"I appreciate it, Lord Rimuru." said Treyni with a smile.
"I'll have to leave things set up so they can be fine during my absence, since I don't know how long I'll be gone." thought Rimuru. "Shion, can you...?"
"What do you wish, Lord Rimuru?" asked Shion quickly entering the office without the human slime being able to finish his sentence.
Rimuru let out a bead of sweat on his forehead at that.
"Inform everyone that I'm going to be absent for a few days." said Rimuru.
"Why? Has something happened?" asked Shion with an arched eyebrow.
"It seems that a friend of Treyni's is in trouble and has asked me to help him." said Rimuru.
"I see, could I go with you on that mission?" asked Shion with puppy dog eyes.
"If it didn't work for me with Milim, with you even less." thought Rimuru with a bead of sweat at Shion's childish plea. "No need. I'll go with Ranga already." said Rimuru causing Shion to let out a pout of disgust. "But I'm counting on you to keep things in order with the others."
"You can count on me, Lord Rimuru!" said Shion with a big smile.
"I know I can, but I really hope nothing happens in my absence." thought Rimuru remembering what happened the last time he was gone for a while.
Later, Rimuru and Treyni went to the former's stall to start the journey.
"Well, everything is ready now. We can start." said Rimuru.
"Of course." said Treyni before spreading her arms and making a bright light surround her and the human slime.
By the time the light dissipated, Rimuru opened his eyes and gaped at his surroundings.
The place they were in was a beautiful floral field with various trees and mountains in the distance, but most striking were the various creatures swarming about.
"Wow. This is the spirit world? It's beautiful." said Rimuru.
"Analysis. The spiritual energy in this place reaches higher levels than normal." said Raphael.
"Well, this place isn't normal." thought Rimuru.
"I'm glad you like it. Even though I've been here before, I never get tired of this scenery." said Treyni.
"And where is that friend that needs my help?" asked Rimuru focusing on the main topic.
"Follow me. I will lead you to him." said Treyni before guiding the human slime through the spirit world.
For a couple of minutes, they were walking through the spirit world and Rimuru was passing the time looking at the scenery around them in passing.
Finally, they both arrived at a small hut located at the foot of a mountain.
Treyni approached and knocked on the door a couple of times.
"Who is it?" asked a male voice of someone who sounded very old from the other side.
"It's me, Treyni." said Treyni.
"Ah, come in, my dear." said the old man giving them passage.
Treyni and Rimuru entered the hut and there Rimuru saw that the only thing in that hut was a table and sitting on the other side was an old man with a white beard, red robes and a hair ornament that resembled a flame.
"I gather that you are the friend Treyni has told me so much about, right?" asked the old man looking at the human slime.
"And you are the friend she told me about. Pleased to meet you, my name is Rimuru Tempest." said Rimuru.
"Nice to meet you. You can call me Roku." said the old man introducing himself.
After the introductions, the dryad and the human slime sat on the floor.
"So, Treyni told me you came from another world and it's in trouble." said Rimuru.
"That's right. I sense that a terrible threat is lurking in the shadows and if not stopped in time, the consequences could be dire." said Roku.
"And that's why you asked for my help?" asked Rimuru.
"Yes. Treyni told me about your skills and exploits and I think you are the right one to solve this problem." said Roku.
"First of all, I would like to know more about the world you come from, since I will surely have to go there." said Rimuru.
"Of course. For starters, in my world there are people with special chi that allow them to bend one of the four elements as well as four nations based on those elements. The Air Nomads, the Water Tribes, the Earth Kingdom and the Fire Nation. However, there is only one person in the world who can bend all the elements. That person is called the Avatar and has the duty to bring balance to the world. He has been in existence for thousands of years, for when an Avatar dies, he is reincarnated into a person from another nation based on a cycle of reincarnation." said Roku.
"Interesting. That Avatar must be very powerful." said Rimuru.
"Well, I was in my time." said Roku with a small smile.
"Wait, you're the Avatar?" asked Rimuru surprised by that revelation.
"I was. Like the seasons, my time came to an end, giving birth to a new Avatar. Avatar Aang." said Roku.
"But if there is a new Avatar, what do you need me for? Can't he take care of that threat?" asked Rimuru.
"Unfortunately, even though I died 100 years ago, Aang is still a child." said Roku.
"How so?" asked Rimuru in confusion.
"For years, the four nations lived in peace and harmony. When he was alive, he was best friends with Fire Lord Sozin. One day, he declared that he wanted to share the greatness of the Fire Nation with the world, but where he saw 'sharing', I saw 'conquest'." said Roku.
"What did you do in the face of that?" asked Rimuru.
"Unfortunately, because of our long friendship, I threatened him to stop his plans and did so for years, but when I was in trouble, Sozin let me die and took advantage of that to carry out his plans of conquest." said Roku.
"It was certainly a mistake on your part. He may have been a friend of yours, but you should have done more than just threaten him." said Rimuru.
"I know and that mistake will haunt me for the rest of my days, but that's not where it ends. Sozin knew that the Avatar was the only one who could stop his plans and knowing that the next Avatar would be born among the Air Nomads, he sent to exterminate them all." said Roku.
Rimuru couldn't help but clench his fists in rage upon hearing that.
Killing an entire nation for just one person was something that bordered on cruel and evil.
He remembered when the kingdom of Falmuth intended to exterminate everyone in his city for business matters, but that was selfishness compared to what Sozin did.
It was then that he realized something he said.
"Wait a minute. If Aang is your successor, does that mean he managed to survive?" asked Rimuru.
"Yes. Aang discovered he was the Avatar at a young age and the pressure of the title was too much for him and he escaped before the Fire Nation attacked. During his escape, a storm suddenly caught him and knocked him into the water. Fortunately, he was able to unconsciously use his Avatar power to encase himself in ice for 100 years." said Roku.
Rimuru was surprised to learn that a child was able to stay frozen for an entire century.
It proved that the Avatar was a being of great power.
"When he came out of his state, it was a hard blow for him to find out that 100 years had passed and that he was now the last airbender in the world." said Roku.
"It must have been a great pressure for him." said Rimuru.
"Yes and more so when I told him that he had to stop the Fire Nation in less than a year. However, with the help of his friends, Aang managed to complete his training and finally, he managed to defeat the Fire Lord Sozin and stop the war." said Roku.
"How exactly did he do it?" asked Rimuru, as he suspected that a child wouldn't be able to finish off the leader in an ordinary way.
"Both I and many others suggested he finish him off for good, but Aang was true to his air nomad ways of respecting all life forms and wanted to find a way to finish off Ozai without killing him." said Roku.
"And he found it?" asked Rimuru.
"Yes, he managed to develop a way to bend the energy inside people, which allowed him to be able to take away Ozai's power." said Roku.
"Does he think that, by taking away his power, it's all over? You can tell he's a child." thought Rimuru. "No doubt Aang sounds like someone strong and brave, but he's still a child and that's why you want me to help him with that threat, right?"
"Yes. He may have ended the war, but he still has a lot to learn and I think, as the leader of a nation, you could help him a little." said Roku.
"So, will you help him, Lord Rimuru?" asked Treyni.
"Was it necessary to ask? If that world is in trouble and you need my help, then I will gladly work with the Avatar to solve it." said Rimuru with a grin.
"I see. I thank you, Lord Rimuru." said Roku bowing in thanks.
"So how do I travel to the other world?" asked Rimuru.
"Don't worry, I'll take care of this. Just because I'm dead doesn't mean I don't still have my powers." said Roku before standing up, as did the human slime.
He held out his hands towards him and closed his eyes for a moment before opening them, revealing them glowing.
Before Rimuru could ask, a bright light engulfed him until he was blinded.
By the time Rimuru opened his eyes, he found himself in a forest.
"Could this be the world Roku came from?" asked Rimuru.
"Analysis. No magiculas detected in the environment." said Raphael.
"That should confirm it. Well, where is Avatar Aang supposed to be? Roku didn't tell me where to find him." began Rimuru before shrugging. "Never mind. It'll be a good way to explore the world and if the Avatar is someone of great power, I'll be able to locate him easily." said Rimuru with a smile before a wing sprouted from his back and he quickly took flight through the sky. "Well, new world, let's see what you have to offer the Demon Lord Rimuru Tempest."