Current scenario: BBEG is being dogpiled by 8 PCs, has a Wish spell, and is insane.
How would the BBEG utilize her Wish?
“I wish this dog pile were actually a pile of dogs.”
Love it. Would that polymorph them?
Either it would polymorph them, or it would make the bad guy THINK they were polymorphed. Depending on level of power and/or what the DM thinks is funnier
PCs snap into a new reality. They are far far from home, on a rocky outcrop above a field of mushrooms and strange creatures. Things don’t work the same way they did back in the real world. Some weapons do nothing, others are insanely powerful, and spells don’t work at all. Creatures and their behavior there make no sense; they may meet other versions of themselves, or forests where you’re underwater when you walk in the forest, things like that. You can’t keep them there for too extended a length of time (it’ll be too unbalanced and not-fun if all their usual gameplay mechanics are just gone for an extended series of adventures), but maybe there’s a type of crossroads with a massive six-legged bear-like creature who says if they each give him one thing that’s most dear to him, he can give them directions closer to their home. The directions take them to some odd astral place that’s at least in the sourcebooks (feywild maybe) and their stuff starts working again, so they can start working on how they might be able to get back to the material plane. If they don’t wanna work with bear guy then they’re going to be stuck in feywild-squared with weird stuff happening to them for potentially quite a very long time.
Or
Fire engulfs the whole stronghold, all of a sudden. Either the BBEG keeps fighting, invulnerable to it, or else gets consumed by it while the PCs have to flee for their lives like the end of Metroid, and the BBEG presumably perishes in the ashes. If you want to make them work for their treasure you can hint to them that they should go back and sift through the ashes and they might find some neat stuff that wasn’t flammable.
Or
Anything the BBEG does to the players starts impacting one of the players (chosen at random) instead of the BBEG. BBEG can hurt them but chooses not to, instead invites them to keep hitting BBEG but they’re only hurting their other party members and BBEG is fine. Once they stop hitting BBEG, BBEG gets really angry and starts chasing them out, telling them to get out get out get out. The next day they discover that they are still getting injured when stuff happens to BBEG, and the new quest is to either figure out how to undo the wish (nigh impossible) or else undo the insanity so that the BBEG will no longer have awful self-destructive things happening to BBEG which will then come back at the party.
Hope these ideas are somewhat useful
I really like 1 and 3. 1 actually fits in really well with the campaign, since the BBEG is from a mirror universe.
I wish, you could see it my way.
Bbeg is now besieged underdog.
deleted by creator
Important clarification: is this the end of the campaign?
No, but it is the conclusion of a major arc.
Obviously you have a lot of possibilities with a villain who is being deliberately portrayed as unhinged. I think the line you need to walk is between making it honest and ruining the fun. One thought that came to mind was the reset. If the bbeg is on the back foot and it looks like the PCs got it, have the bbeg throw a tantrum and wish to try again. Everyone’s HP resets, and maybe he had a few new special abilities to counteract the offensive moves that they first tried. Not to mention, this also goes back to a time when the bbeg has a wish spell. This makes it like a boss fight with multiple health bars, but there are some players who don’t like this type of encounter vehemently and it can feel cheap. I wouldn’t do more than 3 “rounds.” If you party is really tactical, you might also entice them with new weaknesses that are telegraphed or maybe an environmental assistance.
Another idea that came to mind would be wishing they already won. Then you flash forward to the PCs about to be executed in some theatrical way, turning the encounter into a 007 escape that leads directly into the real boss fight. Maybe there are some low level horde waves, some skill checks, some absurd monologuing.
Best of luck!
To be fair, a wish spell is about as close as mortals get to deific power, and IMHO should be treated as such to the fullest extent: complete with reality-bending logic that does not ask for permission and the high likelihood of extraplanar intervention commensurate with the resulting effect (and its consequences).