Phinis Gestor

 is a mage at The College of Winterhold in The Elder Scrolls V: Skyrim.

He is usually to be found in the Hall of Countenance, though he will occasionally appear in The Arcanaeum. He is the college's expert-level Conjuration master. During a conversation with the Dragonborn He admits to practicing Necromancy due to its legality in Skyrim.

Mercantile Information
Phinus sells the following Spell Tomes:
 * Banish Daedra
 * Bound Battleaxe
 * Bound Bow
 * Bound Sword
 * Command Daedra
 * Conjure Dremora Lord
 * Conjure Familiar
 * Conjure Flame Atronach
 * Conjure Frost Atronach
 * Conjure Storm Atronach
 * Dead Thrall
 * Dread Zombie
 * Expel Daedra
 * Frost Thrall
 * Raise Zombie
 * Reanimate Corpse
 * Revenant
 * Soul Trap
 * Storm Thrall

Related Quests
Phinis mentions that Yisra, Ilas-Tei, Borvir and Rundi have gone missing. However, the discussion doesn't create a quest entry. After finding the students and their respective belongings, there's no dialog option to inform Phinis of their fate.
 * Conjuration Ritual Spell

Bugs

 * Sometimes he will not sell certain spells even when your conjuration is high enough. Waiting 48 hours to 1 game week usually fixes this problem (The master-level quests will not be available until the quest to obtain them is completed)
 * (PS3) If waiting doesn't fix the issue, backing up data and re-formatting your HDD works.