Vanishing Twin

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.

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.
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) 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 game mode.
If it becomes a clone of Great Gideon, 2 extra waves of enemies are added to the battle.

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 also not spawn an additional item.
Satanic Bible: Using it 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.
There's Options: Will spawn an additional item choice for the item spawned by Vanishing Twin.
(Tick) Tick: Further lowers the health of both the original boss and its clone.
Vanishing Twin: 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.
- Each clone of Great Gideon adds 2 extra waves of enemies.

BFFS!: Will increase the fetus' size, although it has no effect after the transformation happens.
Box of Friends: No effect.
(IV - The Emperor?) 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 floor's original boss, because the twin won't be active until the next floor.
If (Tainted Lazarus) Tainted Lazarus has used Flip after the boss has been cloned, it will not drop an additional item.