Clavicus appears in Oblivion as one of fifteen Daedric quests available in the game. The purpose of the quest is to fetch Umbra, a sword that can trap souls, and return it to Clavicus. But after accepting his offer a small statuette of Barbas appears in your inventory and he tries to convince you not to retrieve Umbra. Here the Champion of Cyrodil faces a choice and it is up to the player to decide.
The Prince of wishes and deals returns for the fifth installment of The Elder Scrolls. This time around the Daedric quest for Vile is another weapon fetch quest, although this time it is to get the Rueful Axe. Again Barbas comes with you but this time in actual dog form. When the Dovakiin recovers the weapon and returns to the shrine Clavicus offer you a deal. Kill Barbas and keep the axe, or spare him and take the Masque of Clavicus Vile.