Conjuration Ritual Spell

Walkthrough
Upon reaching a Conjuration Skill of level 90, Phinis Gestor teaches the Dragonborn the Summon Unbound Dremora spell, which can only be cast on the roof of the Hall of Attainment. It is advisable to open the Map in the Character Menu and set a marker on the location to help navigate the college grounds. Once summoned, the Unbound Dremora attacks the Dragonborn, regardless of dialogue choice.

Battle
Having a follower present helps, as the Dremora prefers close combat. The follower can serve as a tank, while the Dragonborn casts spells or slings arrows from afar. Casting Banish or Expel Daedra repeatedly drains the Dremora's Health but does not destroy it immediately as it would with lesser Daedra. Without a follower, a Flame Atronach or other Conjured beings can serve as shields while the Dragonborn unleashes lethal Destruction spells. Lightning Bolt or Sparks are advised since Daedra are vulnerable to Shock damage.

Using Unrelenting Force to throw him over the edge of the tower will finish him off instantly. Summon him again, repeat (for a total of three times), and you'll have no problem with the quest.

If you find that you can't summon the Unbound Daedra again before he submits, be sure that you haven't either captured his soul or skipped the dialogue preceding each round of battle.

Aftermath
After defeating the Dremora, the Dragonborn orders him to retrieve the Sigil Stone. Re-summon him, accept the stone, then bring it to Phinis Gestor. Making this quest an active quest helps with locating him.

Phinis then rewards the Dragonborn with a Flame Thrall spell tome. Flame Thrall is a Master-level spell with a base cost of 534 Magicka without the Master Conjuration perk. The conjured being does not defuse with time, but its Health is limited. The "Twin Souls" perk allows two Flame Thralls to be summoned at once.

After this, Phinis offers the other Thrall-spells for a considerable fee (roughly 3,000 each).

The Sigil Stone can also be used at the Atronach Forge, located in The Midden underneath the College. It can be placed on the pedestal, allowing the crafting of Daedric armor, weapons, and hearts.

The Unbound Dremora will stay at the college.

Bugs

 * Followers may attack the Dremora immediately after it is summoned, allowing no dialogue to pass and preventing the Dragonborn from re-conjuring it.
 * Instead of submitting to the player after being defeated, the Dremora may die and be lootable. He will also be unable to be resummoned.
 * If the unbound dremora is summoned during a dragon attack, it may run away and disappear.
 * Occasionally, if you use unrelenting force on the unbound Dremora Lord, it may fall off of the tower, and you will never be able to summon it again.
 * On Completion of retrieving the Sigil stone, returning to Phinis and trying to talk to him may result in a loop of his Staff of Magnus quest dialog.
 * If the Dragonborn beheads the Dremora, and resummons him he may not have a head.
 * As the Unbound Dremora dies it is possible to take his daedric weapon before it vanishes.
 * If the sigil stone is given to Phinis after waking him up he will go back to sleep and you will be unable to complete the quest.