Odren is a priest of Helm and a Knight of the Vigil. He along with the other Knights of the Vigil had the task of keeping the 'Imprisoned One' sealed inside Watcher's Keep. He and the knights, not strong enough to complete the task, asked others for help - or more precisely Gorion's Ward. His real intentions were though to trap Gorion's Ward and the Imprisoned One inside the Keep, as anyone with Bhaal's essence can be a very dangerous creature.
When you arrive at Watcher's Keep, Odren will be waiting directly below the keep. He will ask you to perform the ritual to seal the Imprisoned One in the Keep. After, he will take you to the top of the tower.