Technically, she doesn't disappear, but Blinds you.
I found that fight a big a disappointment. She kept trying to summon a death knight, but I never gave her a chance to complete the spell. Idiot; she summons first, then casts a spell on the knight, then casts Blind... in that order, without skipping any steps. I didn't even lose any hit points. At least she ran away a bit several times, the first 3 Black Ring members stood their ground and repeatedly threw themselves at my sword. I had to re-load and let her complete the summoning to find out what that white sphere starting to form was for.
I had Stun level 5, which helped interrupt her casting. Even without Stun, if you can get the first hit in it really helps. Try running at her when the conversation is over, or Flash Attack if you have it. You can also Blind her first, cast Aura of Command on her death knight, or switch to area effect spells when you get blinded (like Hell Spikes or Deadly Disks). If you cast Curse on her a few times she is susceptible to Freeze, Life Leech, etc. Try scorpions, if you have/use them.
