This covenant can be discovered in the area just beyond the Pontiff Sulyvhan boss room (right before Anor Londo).
Cross through this large, open area which contains several giants and mages and enter the building ahead. Rather than climb the steps inside this building, go the back left corner and attack an illusory wall to reveal a ladder leading down.
The room below contains two Sulyvahn's beasts
and Archdeacon McDonnell, the NPC for the covenant. I suggest aggroing one of the beasts with an arrow to fight them individually. When it's safe, interact with Archdeacon McDonnell to "discover" the covenant.