Way of the Knights by Hateful_Fellow

For example there is an enemy in a tower structure that is randomly named after greece mythology that is kinda out of place. Anyway he joins that sect out of nowhere. Nothing gets explained about that sect then you find out they seal an devil in a prison but devils go out of the prison. Think of it as a dungeon and a lot of people die in to keep the seal on the big bad devil... No one dies... not even fodder.. except enemies... he out of nowhere becomes acting sect leader without doing anything... and more but those are bigger spoilers.

MC basically reincarnates because of something connected to his soul that he found after the kingdom was destroyed. The reason why he died is because after the incident he set out to become stronger and eventually defeat this Abyssal Emperor. When he reincarnates, he is 13 years old (the same age as in TDG(tales of demons and gods)). Goes and finds a secret cave for lost resources of forgotten times. When he gets to school, he meets two of his closest friends, they head to class and once it's finish MC face slaps a student and the instructor. After this debacle, him and his friends go to their dorms, where he explains (in the exact same way as in TDG) of how measuring talents has been incorrect for millennia in the kingdom. He corrects his friends cultivation by given them what's suited to their styles and he too uses one that's just like Nei li's(TDG) - it's overpowered, is difficult to learn, takes longer to breakthrough the realms, and also cultivates spirit, body and mind. That's where I stopped coz I'm not reading it anymore, since I hated TDG and only know so much because of fanfics.