Protector's Robe

Source
Found in a hidden (Perception 35) and locked (Trickery 34) container in the room with hidden (Perception 35) door on the third level of Armag's Tomb. The room is just north from the throne room, where you fight with Armag. You can find this place during The Twice-Born Warlord main quest.

There are also robes for neutral (Arbiter's Robe) and evil (Enforcer's Robe) monks.