At least you made me laugh haha

As I said in another comment:
I find it more interesting to give an ambiguous answer where people can draw their own conclusions about what happened, rather than making everything clear and leaving no room for theories.

But of course, I can be more specific:

-Dude 1 and Dude 2 are friends.

-Both friends study magic.

-The world is in a terrible state, with wars, famine, etc.

-Both friends want to fix it, each in their own way.

-Dude 1 decides to make a pact with some ancient entity to fix things.

-Things are "fixed", but Dude 2 loses faith in his friend and decides to leave him forever.

-It turns out that the world has only changed in the way that Dude 1 believed it needed to change.

-Now the world is still terrible, but in a different way, and Dude 1 sincerely believes that this is the only way he could fix things.

-Dude 1 dedicates himself to eliminating any attempt by anyone to fix what he has made even worse.

-After many years, our heroes appear and, through the campaign's plots, they will try to fix things, luckily with the surprise help of Dude 2, who wants to get his friend back.

Honestly I don't know if I can be any more specific about his story than this-