The Caller

 is an Altmer mage and the leader of the mage faction at Fellglow Keep. She imprisoned Orthorn, after stealing the books regarding the Eye of Magnus.

Strategy
In battle, she prefers to Dual Wield Sparks, but she will occasionally cast Conjure Flame Atronach (or Conjure Frost Atronach or Conjure Storm Atronach [she will occasionally summon more than one at a time at higher levels], if encountered at high level). A battle can be avoided via persuasion, allowing the Dragonborn to take the books and leave. Subsequently, a battle can be avoided by agreeing to hand over Orthorn. With a high level Sneak, the Dragonborn can then approach her from behind and eliminate her.

Quests

 * Hitting the Books