There is a painting in Anor Londo that requires it to access an optional area. You can't leave it without falling off at the end near Priscilla, when prompted I believe, so be prepared. There is a Black Phantom encounter that you should do. The armor is found on the corpse right near the exit of the area, so beat him, then go to Priscilla. It's pretty good, and one of my builds utilizes it. I recommend obviously having patience and that Crest Shield, cause some Pinwheel Skeletons are there guarding a switch I believe, and they make me angry.