I've been working on this campaign for a while now, and have been starting to build up to the bbeg. Think a typical evil spellcaster/dark god. The party will confront him at 20th level, but i want them to have them interact with them before then, so the fight is climactic. The campaign is a homebrew setting, with a mix of high fantasy and darker themes. Any suggestions?

what is his master plan?

When he ascended, he planned to recreate the world for the better. But his general plan was less than desirable for the other deities, who kept reincarnating him into mortal bodies. After hundreds of thousands of years, and learning every little detail of mortals, and being subject to every torture of life, he now plans to kill the gods, and rather than recreating the world, simply destroy it.

The party has technically already met him, though only as one of his mortal incarnations, and he just killed one of the gods, meaning he has a lot more control over his current situation. I plan to have him slowly kill other gods, possibly ones the players also meet at some point, and the world slowly grows more and more unstable as gods of basic concepts and realities die. Think how when Kratos killed different olympians, it gradually caused more problems.

first off, that's SICK (I might use that eventually), secondly, here you go. I assume he's mortal currently, so have him show up in some kind of temple/dungeon and use a scroll of plane shift to get away, (also assuming they don't know his plan) letting them wonder what he was doing there. have them go to a place like mount celestia and find the corpses or various gods. have an adventure on the astral plane and have it be veritably choked with dessicated corpses. have him have an iconic weapon, and it makes distinctive marks on the bodies

Thanks!

I probably should have specified this, but the current mortal form is different than the one the party faced him as. I'm trying to come up with how to introduce a recurring villain while also hinting at the dark god without making them entirely realize until a certain point.