Basically a roguish-warlock-binder (and then some mystic theurge like advancing both binder-warlock, and another the backstab-sneakiness-warlock). Going demilich.
Having the aym (was it?) -personal firewall AND astaroths all abilities (especially blackfire and snake tongue)
Bluff for backstab (and generally lie your way thru the demonlords), eldritch strike with fire..
Basically one heck of a scary headbutting flaming skull :) (hope noone forgot the 10d6 negative +paralysation? ..lol).
And thats not all, theres 3 of them, by ice assassin.. (& semi-frequent wishes to level up the simulcras. Handled by efreti henchmen) and the tripartite hive mind amulet to bind them better into one.