Vanishing Twin

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

Effects

Spawns a 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 (Boss) etc.
The Vanishing Twin familiar will return when Isaac reaches the next floor.

If Vanishing Twin becomes a clone of a boss with a hardcoded item drop, such as Gish or one of the Harbingers, the second item pedestal will be from the boss item pool instead of the boss' regular drop.
Using Satanic Bible before killing the clone boss will cause it to spawn an item from the Devil Room (Item Pool) instead of from the Boss (Item Pool). This item must be purchased just like a normal Devil Deal.
Managing to defeat the boss before the twin has transformed into it will not prevent the twin from doing the transformation and the boss fight will continue.
Obtaining Vanishing Twin in the boss fight itself will not take effect immediately. The boss room must be re-entered in order for the twin to transform.
On floors with multiple bosses (e.g. The Void) or when getting Curse of the Labyrinth, the twin will only act on the first boss encountered.
The twin will not transform into bosses that are randomly found in some default room layouts.
This item has no effect in Greed Mode and will not normally spawn in this gamemode.

Diplopia: 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.
Sacrificial Altar: 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 not spawn an additional item.
IV - The Emperor?: 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 original boss because the twin won't be active until the next floor.
There's Options: Will spawn an additional item choice for the item spawned by Vanishing Twin.
Tick: Lowers the health of both the original boss and its clone.

BFFS!: Will increase the fetus' size although it has no effect after the transformation happens.
Box of Friends: No effect.

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.

Bug!	If Great Gideon is encountered with this item, the fight gets stuck at the second wave because the clone is counted as an alive enemy, resulting in a softlock.

Bug!	Some familiars like Headless Baby will follow the other boss as if it was a familiar. After beating the boss, familiars that where after Vanishing Twin will get bugged and stay at the entrance of every room instead of following the player.

Bug!	Using IV - The Emperor while in a boss fight will immediately kill the twin boss and spawn the second item for free; the player will only have to fight one boss thereafter.