The Night Warden is a ghost that prowls the halls of the Moira Asylum. When patrolling the Asylum he resembles a shimmer in the air and cannot be seen using focus, doesn't react to noise and cannot be damaged using arrows. He spawns together with a document on the wall about him after you pick up the Women's Ward key out of the Men's Ward. This document, called 'Night Warden on duty!" says that you mustn't loiter in the halls, you should close doors behind you and you shouldn't make too much noise. He will attack you when these rules are broken. He seems to instantly kill you, but you immediately respawn on the same place you were killed. He will close doors when he passes them, which is the reason why after you crawled into the room with the Women's Ward key and picked it up, the door of the room from where you crawled will have closed.
The shimmer in the air patrols in a circle around the rooms in the middle of the Men's Ward, and is accompanied by metallic clanking, and occasionally a deep, beastly roar, that can seriously scare any player who isn't aware of his presence. It is uncertain whether you need to be at the shimmering effect after breaking the rules for him to actually attack, or he can kill you anywhere. He never leaves the Men's Ward.
The Night Warden is the only enemy in the game that you can go past without the slightest idea of his presence, though usually players notice something has changed due to the noise he makes. This can cause nasty surprises when he 'kills' you.