Clavicus Vile is one of the seventeen most powerful Daedra known as Daedric Princes. He is best known for granting wishes, often in ways that turn out poorly for the recipient. He is the Daedric Prince of Power, Conjuration, Wishes and Bargains.
Clavicus Vile’s sphere is of wishes and eldritch deals. His dealing with Mundus (the mortal realm) are usually with individuals who wish to have something and, by summoning him, Clavicus Vile gives it to them. However, he is known to take back his deals at inopportune times (for the summoner, not Vile). Why he does this is unknown, though it is more than likely for amusement, as most of the Daedric Princes find mortal behavior humorous (much to the chagrin of mortals).