Duality is an unlockable passive item added in Afterbirth †.
- Whenever a Devil/Angel room spawns after defeating a boss, the other will also spawn.
- Upon entering one, the other one will disappear.
- In rooms with only two accessible walls, only one of the two will spawn.
- Even with Goat HeadA Devil Room / Angel Room Door will always spawn after every boss fight, excluding the first floor and floors after Utero II/Womb II. or EucharistAn Angel Room door will always spawn after every boss fight, excluding the first floor and floors after Utero II/Womb II. 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 JokerTeleports you to the Devil Room or Angel Room. 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.
- One of only four items to appear in both the Angel and Devil item pools, the others being 7 SealsGrants a random Harbinger familiar that changes every 10 seconds., Lil DeliriumGrants a familiar that transforms into another random familiar every 10 seconds., and VoidConsumes all pedestal items in the room. Active items will have their effects added to Void, and passive items will grant stat upgrades..