A sorcerer as well as cleric,   Archdeacon McDonnell is one of the three Archdeacons of the Deep and one of Aldrich's most loyal lieutenants, alongside Royce and Klimt. When Aldrich set off to Anor Londo along with Pontiff Sulyvahn, McDonnell cast off his white crown to stand at Aldrich's side. It is said that McDonnell imparted the spell Deep Soul to Royce and his followers. 
From the Pontiff Sulyvahn bonfire, proceed onward until reaching the building at the end of the big courtyard with several dead Giant Slaves. Upon entering, hit the left wall to uncover an illusory wall. Descend the ladder and dispose of the two Sulyvahn's Beasts waiting at the bottom. Archdeacon McDonnell's remains are located at the far-left corner of the hall, near the Water Reserve bonfire, which will appear once both beasts have been defeated.