Vanishing Twin

Vanishing Twin is a passive item added in The Binding of Isaac: Repentance.

Effects

 * Spawns a flat, rotating fetus familiar that follows Isaac.
 * After entering a boss room, the familiar will move closer to the center of the room, and become a clone of the boss. Isaac must defeat both bosses.
 * The Vanishing Twin and original boss will be set to 75% of their maximum health, and their movements and attack patterns will be slightly slowed.
 * Upon death, the clone boss will drop an item pedestal from the boss item pool, even if the fight isn't over yet.
 * The effect doesn't occur against mini-bosses and end-chapter bosses such as Mom, Mom's Heart, Isaac, Satan, etc.
 * The Vanishing Twin familiar will return when Isaac reaches the next floor.

Synergies

 * : Picking up multiple copies of the item will result in multiple clones getting spawned.
 * Each boss will have its health reduced down further by 75% of the health of its previous clone.
 * Each of the bosses will spawn a separate item.
 * : The twin can be sacrificed normally at any time.
 * If sacrificed after it's transformed into a boss, it will still give an item from the sacrifice, but the boss will not disappear and will also not spawn an additional item.
 * : The twin will clone the additional boss and spawn an item if the original one has not been visited yet. However, this will not allow for another item to be obtained from the floor's original boss, because the twin won't be active until the next floor.
 * : Will spawn an additional item choice for the item spawned by Vanishing Twin.
 * Tick: Further lowers the health of both the original boss and its clone.

Interactions

 * : Will increase the fetus' size, although it has no effect after the transformation happens.
 * : No effect.
 * If has used  after the boss has been cloned, it will not drop an additional item.

Trivia

 * A Vanishing Twin is a fetus that dies during gestation and is reabsorbed by its twin, hence the appearance of the item.
 * The familiar's flat appearance is due to a real life phenomenon known as fetus papyracaeus, where instead of a vanishing twin being physically reabsorbed by the other, the body stays in the womb and gets crushed and flattened by the healthy twin in full term between the fetus' expanding membranes and the uterine walls.