Tamriel:Clavicus Vile

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Clavicus Vile is the Daedric prince whose sphere is the granting of power and wishes through ritual invocations and pact. Clavicus has a companion named Barbas. While capable of selective shapeshifting, the form it assumes most of the time is that of a dog. Clavicus himself is depicted as a jovial fellow with horns protruding from his forehead.

Clavicus Vile has played a role in many of the Elder Scrolls games:

It is possible that Clavicus Vile assisted Jagar Tharn in his temporary procurement of the Imperial throne, though by no means certain. In the past, Clavicus Vile has worked through mortal representatives to gain what he desires.
The Hero of Daggerfall received the Masque of Clavicus Vile in exchange for assassinating either a rogue Wereboar or Werewolf. In Daggerfall, Clavicus' enemy is Ebonarm and his summoning day is the New Life Festival.
In Redguard, Clavicus gambled with the protagonist Cyrus for the soul of Cyrus' sister against Cyrus' own soul.
In Morrowind, the player may obtain Clavicus Vile's artifact, the Masque of Clavicus Vile, by defeating Sorkvild the Raven, a Nord sorcerer who has been terrorizing the village of Dagon Fel.
In Oblivion, his Masque can again be obtained, this time by finding and returning his sword Umbra.