>>538795173
Ookay
Ending 1 - Asuka kills Satoshi (never stated if this was in self-defense or he did it because he's becoming heckin' evil), decides not to go to the Makai to look for answers, so the portal closes and their journey effectively ends there. Big mistake, Satoshi becomes Ahriman, Asuka kills him again, the stage explodes and he wakes up in a forest and hallucinates Yggdrasil absorbing his brother. (I say hallucinate but it actually happened)
Ending 2 - Goes to the Makai, kills the three leaders of the demon war, goes back to Aeacus and decides to become Ahriman. Ahriman kills all of his buddies and starts crying over their deaths. The guide actually clarifies that he's not crying because he's sad but because he's finally able to understand how fragile human life is. Then he makes a new contract before nuking the universe, which will reset the world back to before the big bang, and it's kind of implied that the events of Ronde will eventually play out again, whether exactly the same or with some differences, I don't know. When I scan the guide I'll go over it again.
Ending 3 - Asuka doesn't reincarnate into Ahriman, so Sawaki randomly does. It comes completely out of left field but this was actually implied way earlier in the game, it's just so subtle and the game doesn't focus on it at all. The guide also says he did it because he wanted the power of the demons. What an ass. So Asuka kills him and Satoshi comes back to life, happy ending. This actually is completely unexplained even in the guide. He's just alive because he is. And it's explained there that even though Ahriman didn't nuke the universe this time, he eventually will.
Then there's the true golden ending where Asuka kills Ahzura Mazda. That's it. You don't get a special cutscene or anything, just an item saying 'good job, you beat the superboss'. He's not that hard if you plan ahead but he does resist everything iirc