Rotgut

Rotgut is a boss added in The Binding of Isaac: Repentance that can appear in.

Phase 1
The first phase occurs against Rotgut's "face" in its regular boss room. Rotgut is stationary in the center of the room and performs the following attacks:
 * Spews a constant wave of projectiles that alternates above and below it in a fanning pattern.
 * Spews out waves of fluctuating spaced-out projectiles in a disorganized pattern.
 * Fires rings of 8 expanding projectiles that alternate between rotating clockwise and counter-clockwise.
 * Fires three waves of projectiles in a row:
 * A radial of tightly knit slow-moving tears.
 * A radial of spaced out fast-moving tears.
 * A burst of randomly positioned tears that hover for a moment before dropping.
 * Coughs up a horizontal burst of red projectiles to the left and right side of itself.
 * Vomits out projectiles, bones, as well as Globin goo, Gazing Globin goo, and Small Maggots that rain down in a large area.

Phase 2
Once Rotgut's first form has been defeated, it sucks Isaac into its mouth and into a Crawl Space-like interior. The second phase consists of a large maggot that sits in the bottom right of the room and performs attacks depending on Isaac's position.

If Isaac is on the platform the maggot uses its rear end:
 * Launches a cluster of projectiles onto the platform and covers it in green poisonous gas.
 * Launches an shot that bursts into arcing tears.
 * Shoots out 6 slow travelling tears that travel in a wave pattern across the entire room.

If Isaac is below the platform and on the ground, the maggot uses its head:
 * Spits out a stream of arcing blood projectiles along with a few Attack Flies and Small Maggots.
 * Shoots out a laser.

Phase 3
Defeating the Maggot allows passage into the next chamber, which contains Rotgut's "heart". The heart switches randomly between two modes of projectile patterns that occur for the entire duration of the fight:
 * Mode 1: shoots out half-rings of 6-7 green projectiles below it with each pulse of the heart.
 * Mode 2: drops 3 yellow projectiles in random locations with each beat of the heart. Each projectile splits into 2 arcing projectiles after hitting the ground.

The heart can perform additional attacks that occur independently of its two set modes: After defeating this phase, Isaac is automatically ejected out of Rotgut's insides and back into the boss room.
 * Retreats upwards and spawns Leeches on the end of the room furthest from Isaac. These Leeches move quickly across the screen and despawn upon reaching the edge of the screen. The final Leech spawned has a chance of being an Adult Leech.
 * Drops 2 large blood projectiles below it one after another. Each bursts into smaller projectiles that bounce all over the room.

Trivia

 * Compared to Antibirth, Rotgut's 1st and 3rd phases have a new visual design.
 * Rotgut's appearance change was a decision made by Edmund.
 * The maggot inside of Rotgut has been nicknamed "Gus" by the developers.
 * Tainted Spitty, an enemy which can appear during the Ascent to reach, bears a strong resemblance to Gus in both appearance and attack patterns.
 * Rotgut is the informal name for a grade of moonshine that is very poor quality, being poor enough to be deadly to drink.
 * Rotgut is the name of a level from Super Meat Boy, one of Edmund McMillen's earlier games.
 * Rotgut's current depiction is explained by a developer to be possibly a huge Fatty fused to the floor, it also might be a variation of It Lives.