The Stain

The Stain is a boss added in The Binding of Isaac: Afterbirth that can appear:

In all environments of Chapter 2 (Caves, Catacombs, Flooded Caves).
Only in the Caves and the Flooded Caves.

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:

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 2 Chargers and can have a maximum of 3.

It will stay vulnerable for a brief moment after performing an attack before quickly burrowing underground again.

Champion Versions

Grey: Continuously spawns 2

Round Worms instead of Chargers, with a maximum of 3 at any time. Never fires barrages of bullets.

Gallery

The Stain performing its barrage attack.
The Grey Champion Stain.
The Stain as one of Delirium's forms.

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.
The Stain has an alternative more degraded variation known as The Pile.
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/ five bosses that can be heard in the versus screen; the other are The Forsaken, Big Horn, Mom, Mom's Heart, It Lives!, 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.