A campaign where the final boss controls the DM.

So, my idea is complicated and niche. The main purpose of the idea is that the BBEG has discovered that their life is governed by the ideas made real of a being beyond their plane (the DM) and that everything conforms to their peculiar rules. He attempts to wrest control of this universe and the DM recruits other outer planar beings to send in "champions" to stop him.

The champions are the PCs, and none of them know the reason behind the need to fight the bbeg, the Players don't know either. The PC's are gathered by a prophet, who foresaw a future in which the BBEG becomes the ruler of the planes. This is, the DMPC.

The story could consist of clever manipulation of places, rolls, enemies, like a simulation beginning to break free.

The BBEG ends with him taking control of both the DM and the DMPC, some clever rules manipulation, errata being ignored and the players at a disadvantage, not in strength, but the world and laws of the world are against them.

TLDR: BBEG who manipulates the world to ignore or bend rules like a peasant rail gun, and controls the DM for a climactic finish.