Nope.
When I get to the tunnel where she's located with the pentagram, I do talk to her. She says "You...Morigaine? Yes. Morigaine. I see it. You are here. Good. Now kill me. Kill me! Fast! Hurry!
And then no matter what I say...
1"No, you must come back with me.
2 "Why?"
3. OK! (Kill her)
...she responds by telling me it too late and she leaps into combat mode. No spawning horrors. Nothing but her attacking me while I peg her with an arrow now and then to see if that'll bring on the battle.
I'd rather not kill her, but if I have to I guess I have to. Maybe I left out something I was supposed to do first...?