The Beast

The Beast is a boss added in The Binding of Isaac: Repentance that appears in and is the final boss of Ending 22 and The Binding of Isaac as a whole.

Behavior
After defeating Dogma, Isaac is given, granting him flight and allowing him to traverse The Beast's 2D Crawl Space-like arena. also grants a one time shield, the effect of  (if playing in Normal Mode), +0.1 speed and +2 damage, and heals him up to 6 hearts with red and soul hearts.

At the start of the fight, the Beast sits inactive in the background. Flying around the Beast are four Ultra Harbingers which attack Isaac one at a time in the order of Famine, Pestilence, War and Death. Once they've all been defeated, the Beast lets out a roar, submerges into the lava, then reemerges on the right side of the screen, beginning its fight.

Phase 1
During its first phase, the Beast uses the same sequence of attacks indefinitely:
 * Charges towards Isaac, while harmful, destructible stalagmites and stalactites move towards Isaac.
 * At the end of the charge, the Beast begins to inhale large rings of fire which have one small gap that Isaac can fit through.
 * Any remaining stalagmites or stalactites on screen are destroyed at the start of this attack.
 * After inhaling 8 rings of fire, the Beast stops and fires a colossal -like beam that covers the bottom and middle portions of the screen.
 * After this attack, it performs a quick charge across the screen before repeating the initial charge again in the opposite direction. Similar to the -like beam directly before, it covers a large portion of the bottom and middle parts of the screen.

Phase 2
The Beast enters its second phase at approximately 50% HP and loses its inhaling attack, but gains 2 new ones. Additionally, during the charging attack, some stalactites are cracked and fall when Isaac passes underneath.

At the end of a charge, the Beast performs one of the two attacks:

After using either of these attacks, the Beast submerges into the lava and causes glowing orange lines to appear beneath Isaac and in other random areas. The glow of these lines increases in intensity before launching out a large fiery projectile that returns back into the lava. The Beast gradually increases the rate at which projectiles are launched before eventually reemerging from the lava on the opposite end of the screen and launching a small burst of large fiery projectiles arcing in all directions.
 * Opens its mouth and fires 3 radial waves of slower-moving fire, then firing 6 radial strings of fast-moving projectiles which travel through the gaps left between each fire.
 * Roars and spawns 4 Beast Souls which chase Isaac and charge at him trying to predict his movements before dissipating.
 * Any remaining stalagmites or stalactites on screen are destroyed at the start of this attack.

Phase 3
The Beast enters its third phase at approximately 33% HP and is indicated by a burst of blood. The Beast is forced to submerge into the lava and reappear on the other side of the screen, then charging towards Isaac indefinitely while more stalagmites and stalactites spawn behind him. Cracked stalactites do not appear during this phase.

Once the Beast's HP reaches approximately 2000, its horns crack and its body ruptures in boils. It degrades through two more stages: losing 2 horns and an eye once reaching roughly 1000 HP, and losing another eye, horn, all of its teeth and generally having a melted look once reaching roughly 500 HP. Upon defeat, a large light ray comes from above and strikes it down, followed immediately by Ending 22.

Damage Scaling
Damage scaling is an unclear mechanic introduced by Nicalis in The Binding of Isaac: Afterbirth to provide a more challenging experience to the players, enabling some enemies to retaliate against high damage attacks by deploying an "armor" of a sort to greatly reduce the amount of damage taken. Further details and an in-depth explanation may be found here.

Trivia

 * The Beast is heavily based on the first Beast described in the Book of Revelation.
 * The Beast's theme is called "Revelations 13:1", referencing the section in the Book of Revelation describing the first Beast.
 * A narration performed by Matthias Bossi reads parts of this passage during the cutscene prior to The Beast's fight. This narration says, "And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority."
 * The Beast is the only boss to be preceded by an introductory cutscene rather than a VS screen.
 * The cutscene depicts Isaac being crushed by a giant static cross that rises from the remains of Dogma, which turns his tears into static. Following this, the room begins collapsing, revealing a hellish cavern while Isaac sprouts a pair of wings and flickers between resembling, , and . Isaac then flies up to face The Beast rising up from the lava.
 * Isaac's flickering between ???, the Forgotten, and the Lost is a reference to the events of Ending 20.
 * The Beast is one of the only 2 bosses to take place in a Crawlspace-like side-scrolling environment, the other being Rotgut.
 * At the end of the fight, Isaac's father refers to The Beast as "the Mother of Harlots".
 * The Beast appears to be wearing a tattered version of Mom's dress, suggesting that it is yet another interpretation of Mom from Isaac's perspective.
 * When using the "I'm Drowsy..." pill during the battle with the Ultra Harbingers, there is one extra slow sign in the position of where the Beast will be, indicating that the Beast is actually "hidden" below the lava before the Ultra Harbingers are defeated.
 * Revelations 13:1 is the longest track in the entire Rebirth OST, sitting at 7 minutes and 53 seconds. This makes sense, as the theme is highly dynamic, changing depending on where the player is in the fight.
 * During the section that plays while fighting The Beast, Latin chanting can be heard in the background sometimes. The chanting is "mori bestia, cum nostra lucis", which translates to "die, Beast, to our light".
 * In Revelations it is said that the Lord is who defeats the Beast, which may explain the beam of light that comes down to kill The Beast.
 * If the Beast is affected with any Status effects as it dies, the beam of light changes color. This is likely because the beam of light is part of The Beast's frames and not a different entity entirely.
 * Enemies turned into poop with E. Coli or other means affect the game's scrolling, as the poop will remain on the screen regardless of whether or not the screen is scrolling.
 * In the fight's background, each of the four Harbingers can be seen doing a small animation before they fly into the foreground. Ultra Famine chuckles, Ultra Pestilence raises his fists in the air, Ultra War becomes enraged, and Ultra Death facepalms before brandishing his scythe.
 * They can also be seen doing a small animation whenever Isaac takes a hit. Ultra Pestilence waves his arms and grins, Ultra War raises his arms and then "slams" them down, and Ultra Death laughs. Ultra Famine doesn't have one because he is the first Ultra Harbinger to come into battle.
 * According to the game files, the Beast's boss fight takes place in Hell itself.
 * Despite being in a standard fire-and-brimstone depiction of Hell, Isaac's last will still retains Home's "some cold place" location line, suggesting some level of illusion to the events of The Beast.
 * The Beast is most likely female, due to its nickname being "Mother" of Harlots.
 * The 4 Beast Souls that The Beast spawns may be a reference to The Binding of Isaac: Four Souls, a card game created by Edmund Mcmillen.
 * The Beast was teased initially in a post made to Steam by Edmund 5 days before Repentance's release, which included an artwork that featured its faint silhouette in the background.
 * The skull of The Beast can be seen in the icon for the achievement.