To get Sharn Gra-Muzob to teach you a necromantic spell (summon Ancestral Ghost or Summon Skeletal Minion, if I'm remembering correctly), you have to confront her with her necromancy. Without too much spoiling, look close at "things" that belong to her, and when you find something that says "Necromancy" to you, show it to her...
This requires decent levels of theiving skills...
I may also be wrong, so quicksave
