Duality 
Effects
- Whenever a Devil/Angel Room spawns after defeating a boss, the other will also spawn.
- Upon entering one, the other one will disappear.
- Both rooms will spawn even if a devil deal was made before.
- (Removed in Repentance) In rooms with only two accessible walls, only one of the two will spawn.
Notes
- Even with
Goat Head or 
Eucharist both rooms will not stay open, and entering either of the rooms will make the other disappear.
- If Duality was bought through a Devil deal, it is still possible to get Angel Rooms as well as Devil Rooms.
- Teleportation via a (Joker) Joker card, a Red Chest or a Sacrifice Room before defeating the boss and choosing a door will follow the normal rules of Devil/Angel Room generation, instead of a 50/50 chance.
- The room that was chosen this way will be the only one that opens once the boss is defeated.
- Teleporting after choosing a door will always take you to the same room.
Synergies
- (Added in Repentance)
Glowing Hour Glass: Time can be reversed while inside one deal to go into another.
Interactions