It is one of three possible powers gained from the quest Black Book: Untold Legends.
"'Summons a Dremora merchant for 15 seconds."
When activated, a Dremora Merchant appears and offers to trade much like a regular merchant. He will buy any type of item and sells Daedric armor and weapons, however at lower levels he may only sell Ebony armor and weapons. He typically has 2,000 available for trading, which resets after two days. He offers a way to purchase Daedric armor or weapons before reaching level 48.