Copy-pasted from another post of mine, in which someone else asked a similar question:
One of my favourites was a fellow called 'Mother' Manning (real name Mariano Manning). He started out as a slightly-reclusive sterile druid down on his luck, unloved by all the women of his local area, and desperate for a family of his own. He reached out to the gods, and Lamashtu answered.
The Mother of Monsters filled him with fiendish Thriae larvae, turning his body into a living incubator of sorts and making him essentially the queen of a hive of fiendish thriae devoted to Lamashtu (because Thriae CRs are ridiculously widespread, and the PCs couldn't deal with a true queen). When the PCs first faced him he was a rather creepy NE human druid with waxy holes in his chest, some filled and nurturing growing larvae, some of which would occasionally burst as a larva wriggled out of his body, and some of which gaped open from where larvae had already left. When he was killed, the fiendish larvae still remaining in his body amassed in desperation, causing his head to burst open in a mass of flailing fiendish tentacles, effectively turning him into a Spawning Canker (Adventure Path #46) associated with Lamashtu rather than Shub-Niggurath.
Managed to give 3 out of 4 players nightmares with that one.
My BBEG is going to be a Half-Celestial Aasimar who's a Paladin multiclassed with an Antipaladin, who's sheer confidence and self belief prevents him from falling, and this is slowly undoing the fabric of reality. The PCs will be on a race against time to get strong enough to take this sucker down.
Kinda got shades of Dogma going on.
There ya go.