This screams of darts and poisons, and maybe a warehouse complex. Maybe I'm wrong, but this sounds like the kind of BBEG who is a sadistic bastard that enjoys playing with his victims, eroding their resistance and morale, before finishing them off in a cold and bloody way. This is the kind of BBEG who will have his lair littered with bodies hanging in spots where the PC's will find them and be scared, who will constantly taunt them seemingly from nowhere, and who will chip away at them slooooooowly, until they are vanquished at his feet.

Amirite?