The Stain

The Stain is a boss added in The Binding of Isaac: Afterbirth that can appear:
 * In all environments of Chapter 2.
 * Only in and the.

Behavior
The Stain burrows underground when Isaac is not within range. While burrowed it becomes invincible and follows Isaac's movements. When close to Isaac, it pops out and performs one of the following attacks: It will stay vulnerable for a brief moment after performing an attack before quickly burrowing underground again.
 * Sends out 2 patterns of numerous projectiles in all directions. The first barrage consists of fast-moving bullets that slow down. The second barrage consists of slow-moving bullets that speed up.
 * Sends out tendrils to its left and right leaving small puddles of damaging Red Creep directly around The Stain and to its either side.
 * Spawns two Chargers and can have a maximum of two.

Champion Versions

 * Grey: Continuously spawns Round Worms instead of Chargers, with a maximum of two at any time. Never fires barrages of bullets.

Arena
When fighting The Stain following obstacles can sometimes spawn:
 * 2 red poops (near upper-right and bottom-left corners)
 * 2 red poops (near the center)
 * 4 red poops (in all corners)
 * 2x2 Pits in upper-right and bottom-left corners

Trivia

 * It is the posthumous version of Polycephalus. It appears that the top heads have been stripped of their skin, while the bottom head only lacks its first skin layer.
 * Unlike its alive counterpart, the lower head has closed eyes and eyelids.
 * 2 Stains can spawn for the nightmare wave for the 6th floor of Greed Mode.
 * A third head can be seen in the background in The Stain's boss art.
 * During the versus screen, The Stain's popping out sound can be heard. The Stain is one of seven bosses that can be heard in the versus screen; the other six are The Forsaken, Big Horn, Mom, Mom's Heart (pre-Repentance), It Lives (pre-Repentance), and  Mother.
 * Prior to Booster Pack #5, The Stain could spawn Drowned Chargers if it was fought in the Flooded Caves. This also applied to Chub.
 * The Stain has an alternative more degraded variation known as The Pile.

The Stain