To answer your question, sometimes I use 'dramatic flare' for such villians, just to prove a point. You could have one of her servants interrupt their conversation, at which time Iggwilv points a finger, a green hellish ray strikes them in the chest, and the PCs watch as the servant's body and soul are shredded into a million pieces, without so much as a blink from Iggwilv. Callous, cold, unfazed...
Back in the day we ran the adventure where we went after Iuz's vampiric sister. We won, but ran into Iggwilv at the end. My naive elf told her off, at which point the DM zapped him dead, enslaved his soul, and I still think his ghost is serving Iggwilv to this day. But that one act caused the entire party to run like hell, never look back, and taught us to NEVER tempt powerful bad guys/gals like that again!