Mirror Room

The Mirror Room is a special room that always spawns in Glacier and Tomb. It is where the player fights Narcissus in Glacier and Narcissus 2 on Tomb. The floor of the room is reflective, showing the player, the boss, tears, and any enemies that spawn. You are unable to bomb out of the boss fight once you are in, so make sure you are prepared!

The Mirror Room is not visible on the minimap, and the minimap view in the Mirror Room shows a blank. The Mirror Room, the Narcissus bosses, and the mirror items (Mirror Shard, Mirror Fragment) ties into Chapters Two and Three of Revelations to create a multi-item, such as Key Piece 1 and Key Piece 2 are needed to fight Mega Satan. But as Chapter Three has not yet been released, the only benefit for going into the Mirror Room is the Mirror Shard and Mirror Fragment items.

Looking into the mirror and facing one's self seems to be the theme for this boss fight. For more extensive information on the Narcissus Boss fight that takes place in the Mirror Room, see Narcissus and Narcissus II.

Accessing
In any Revelation floor, a mirror door can be found glowing in a random room.

An explosive force such as a bomb is then needed to destroy this mirror door. An ominous sound is then played to warn the player of the danger within and thus signifies access to the room.

Trivia

 * Once the boss within is defeated, the floating mirror pieces seen outside the entrance will lie on the ground motionless to show that the boss has been conquered.