Warning: Spoilers ahead
Uldred is a senior mage of the Circle of Magi. He was at the Battle of Ostagar with Wynne, but escaped unharmed and returned immediately to the Circle of Magi to tell them of the Grey Wardens' treachery and to convince the Circle to join Teyr Loghain's cause.
 Circle Meeting
The Teyrn's true treachery is revealed to the Circle when Wynne returns to report on what she witnessed. First Enchanter Irving, Niall and other members of the Circle convened a meeting in the Harrowing Chamber to question Uldred. During the meeting Uldred wished to leave, but Irving and others refused to let him go. Uldred then casts a spell to throw the other mages against the wall, and Blood Mages rushed into the Chamber. Uldred also tried to summon demons to his aid, but was overpowered by one of his summoned demons and became an Abomination. He then started to turn his fellow mages into Abominations.
 The Fight
Uldred turns into his true self as a massive Abomination with various powers include spells and physical attacks. He is capable of a powerful Crushing Prison attack that can last 6 seconds or more. He will also occasionally start to call upon a demon to inhabit the body of one of the fallen mages in the Harrowing Chamber, turning them into powerful Abominations. By the time you reach this fight you should have the Litany of Adralla - use this when you see a large glowing circle begin to form around the fallen mages and it will prevent the abominations from appearing.
 After the Fight
Uldred drops Cinderfel Gauntlets (mage only) with +20% fire damage.