Conjuration Ritual Spell

Initiated once the Dragonborn reaches Conjuration level 90, Phinis Gestor teaches the Summon Unbound Dremora spell, which can only be cast on the roof of the Hall of Attainment. During the quest, the Dragonborn is required to defeat a Dremora. Although many tactics for destroying the being are available, Daedra are vulnerable to shock damage, thus entering the battle with lightning magic, weapons imbued with electricity, and Dragon Shouts such as Storm Call, damage output is optimized and the battle takes less time. Once summoned, the Unbound Dremora attacks the Dragonborn, regardless of the dialogue choice. Defeat the Dremora to unlock access to the highest level Conjuration spells in Skyrim; spells that summon thralls with no expiration.

Battle
Having a follower present helps, as the Dremora prefers close combat. The follower can serve as a decoy while the Dragonborn casts spells or shoots 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. Having the Oblivion Binding perk and using bound weapons on the Dremora does not banish it as it would any other Dremora or Atronach. Without a follower, a Flame Atronach or other Conjured beings can serve as shields or distractions while the Dragonborn unleashes lethal Destruction spells. Lightning Bolt or Sparks are advised, since Daedra are vulnerable to Shock damage.

Unrelenting Force may be used to throw him off the tower which will finish him off instantly. Summon him again and repeat twice more to finish the challenge.

If the Unbound Dremora cannot be summoned again before he submits, be sure that his soul has not been captured and the dialogue preceding each round of battle hasn't been skipped.

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.

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 remains at the College.

Trivia

 * The Dremora's soul can be trapped in a Black Soul Gem without stopping the quest.
 * Daedric weapons can be looted from Dremora's corpse if it can be opened before it disappears.

Bugs
Ритуальное заклинание Колдовства (Квест)