After his death Tian Liu found himself in what seemed to be like a chamber. There were brick stone walls surrounding him and shelves of books in between. Tian Liu had never seen a place like this in his life and he thought it wouldn't make sense for him to be reborn here.
"Welcome dear cultivator," an old man with a white hair and a very long beard said.
"Oh hey." Tian Liu greeted the old man, "Where are we?"
"I mean look around doesn't this tell you enough?" the old man said. "I like to call it the Side Effect if that's what you're asking."
Tian Liu put two and two together, "Who are you?"
"Shouldn't it be obvious? I'm the Immortal Demon King himself."
"This is the side effect of drinking your essence?" Tian Liu now with a slightly hunched forward posture now said.
"Sure is! 100,000 years left."
Tian Liu was silenced.
"You're joking right."
"No, you have to spend 100,000 years here, with me."
Tian Liu walked to the corner of the chamber and sat there. He began thinking what it would be like to spend 100,000 years with some crazy old man and immediately wished he would have just died.
'Basically living the average person's life for 1000 times. Just sitting here doing nothing except talking to the old man. I might be able to learn a thing or two from him, he is the Immortal Demon King after all.'
That's when Tian Liu noticed the books on the shelves. He thought nothing of them before but when he looked closely he realized these are books about martial arts and cultivation, and not just any books there were some pretty rare and useful books in the collection.
"Interested in books?" the Immortal Demon King said.
"Not really much else to be interested in here," Tian Liu remarked. "Hey old man, can you teach me some martial arts?"
"Ah you're the first person to ask that since I've made this."
"There were other people who drank the essence?"
"No, you're the only one so far you left some essence behind right?"
Tian Liu shook his head.
A aura of qi came around the old man, "You didn't leave any essence behind?!"
Tian Liu shook his head again.
"You know how hard I worked to obtain that essence just for it to only be wasted by one person! After 100,000 years I'm gonna be so lonely."
Tian Liu thought for a second, "Teach me martial arts and cultivation."
"After what you did I won't be teaching you."
"I have a good deal for you."
After hearing Tian Liu's deal the Immortal Demon King chose to accept. Tian Liu's offer was that he would make his essence when regressed so the Immortal Demon King wouldn't be lonely anymore in trade for the Immortal Demon King teaching him everything he knows about martial arts and cultivation.
It was a pretty good deal considering Tian Liu didn't have anything valuable to offer.
So Tian Liu began training. Even with his mediocre talent anyone knows he will become dangerous after 100,000 years of training.
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"It took you 1000 years to master what I mastered in 100 years, Tian Liu. Your talent is mediocre at best. Are you sure you want to become a cultivator? Even if you're as strong as me right now your qi core will still have a very low purity."
"I have 99,000 more years."
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10,000 years later and Tian Liu was winning against the Immortal Demon King at least half of the time. When the Immortal Demon King wasn't training him, Tian Liu was consuming all the books there. He applied some of the martial art techniques into his sparring sessions and it worked less than half of the time.
"I'm getting old. Training you is really getting to me," the Immortal Demon King said while panting with his hands on his knees. "Once you get back to the real world you have to train your body so it can keep up with your techniques, also focus on getting your qi core rank up, even with little talent you should be able to get to at least rank 5 with the techniques I've been teaching you. Also make sure to make the essence for me, if you don't I'll really kill you."
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At the 50,000 year mark Tian Liu was consistently beating the Immortal Demon King. Even though his qi purity was much lower he had a knack for martial arts (and 50,000 years of hard work). He kept rereading the best martial art books he could find and mastering the techniques. He cultivated during his stay knowing it would be useless once he got out. He just wanted to actually apply the cultivation techniques and see how fast he could get to each rank.
He mastered the best cultivation techniques he could find on that shelf, and he even mastered the worst ones too. Though this didn't fully make up for his lack of talent, it helped a lot. He also trained his body during his stay. Even though it won't be brought back to when he gets regressed he thought it would be good to build some mental fortitude while doing it.
"You're getting good Tian Liu. After 50,000 years of training you finally surpassed me. I learned a thing or two from you seeing your martial art skills. Let's have an all out battle."
The Immortal Demon King never really showed his true power to Tian Liu and today he finally did. When he unleashes his true power Tian Liu found he could kind of keep up. Though he knew he couldn't win he found some openings and landed some good shots.
The Immortal Demon King also got stronger during Tian Liu's stay. After all, sparring daily and instructing a student really does something. They say teaching is the highest form of learning.
But Tian Liu managed to keep up, which made sense since he was putting 100 times the work the Immortal Demon King was putting in. Not like the Immortal Demon King was trying to get stronger but at the 50,000 year mark he felt the passion of the fight from the Immortal Demon King.
They became really good friends as they were the only people each other could talk to there. They exchanged stories and shared laughs together.
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At the 99,000 year mark, Tian Liu was easily beating the Immortal Demon King even with him revealing his true power. At this point they were more like training partners as the Immortal Demon King was also trying to get stronger during Tian Liu's stay.
But Tian Liu was getting stronger faster, as he was training much more.
He mastered every technique he could find on the shelf and even mastered miscellaneous skills that he thought would be useful. He hoped this would make up for his below average qi purity but the Immortal Demon King assured him that 100,000 years of hard work is 1000 times more than world class talent.
In his final year he said goodbye to the Immortal Demon King. It was a bittersweet moment for Tian Liu as he was leaving his best friend after 100,000 years of bonding. But he knew the future he had in store with him was going to be good. After all, he still hasn't forgotten what the disgusting bastard Gao Wan did to him.
He was going to make him suffer.