[Suzerain of Sheol]

She has three forms you have to fight in a row like the Nameless King, the first one is pretty fair, she's easily stagger-able and her only real trick is going invisible and trying to backstab you. The second form adds a second boss to the encounter, but they share a lifebar. Once you learn the patterns of the new boss, it's pretty simple to bait him into a corner and devastate his health.

But the third form. *shudders* She gets resurrected by the abyss-flame and just goes completely insane. Every single hit is practically a 1-shot and she's insanely fast and hard to read. And she has this execution grab where she basically just scissorses your head off with her two scythes that is... admittedly, worth seeing at least once. :P

I still want to beat her on an up-close character, but I haven't managed it yet.

Oh, and in all of her forms she has this ridiculously-huge ice field aoe that frostbites you and then explodes for massive damage. It's super annoying to deal with.