Conjuration Ritual Spell

Overview

 * Prerequisites: Conjuration level 90.
 * Quest Giver: Phinis Gestor
 * Reward: Ability to cast Master-level Conjuration spells.

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 is 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 and do the same three times and you'll have no problem with the Quest.

Aftermath
Defeating the Dremora, the Dragonborn orders him to retrieve the Sigil Stone. Re-summon him and accept the Stone. Bring the stone to Phinis Gestor. Making this quest an active quest helps with locating him.

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

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

Bugs

 * There is a bug involving the use of followers in this quest. Followers tend to attack the Dremora, immediately after being summoned, allowing no dialogue to pass and preventing the Dragonborn from re-conjuring it. In case of Barbas it is recommended to disable him untill the quest is done, since the dog cannot be ordered to stay at one place.
 * If you loot the Dremora Lord after killing him you will be unable to resummon him and will have to load from a previous save to be able to continue the quest. However if you are quick enough you can grab his weapon.