The Fallen

The Fallen is a boss that can appear on any main path Chapter, with the prerequisite that Isaac has entered (not just encountered) a Devil Room through any methods, and with the following exceptions:

It cannot appear on floors with predetermined bosses such as Mom, Mom's Heart, Isaac, etc.
It also cannot appear on any of the alternate Chapters added in Repentance.

It is also encountered as the first phase of the Satan battle.

Behavior

Phase 1

The Fallen moves randomly around the room in cardinal directions and can perform the following attacks:

Fires 3 projectiles in a spread followed by 4 projectiles in a spread in succession towards Isaac.
Increases its speed and chases Isaac. After taking a certain amount of damage, it halts and fires a Brimstone laser in each cardinal direction.

Phase 2

At 50% health, The Fallen splits into two smaller versions of itself. The two clones have identical behaviour.
- It is possible to avoid the split if the Fallen takes a single hit doing more than 50% damage that causes poisoning or burning as the final one.

The Fallen deals a whole heart of contact damage in both phases.

Gallery

The Fallen in-game.
The Fallen as one of Delirium's forms.

Notes

Unlike other bosses, the item pool of the Fallen room is the Devil Room item pool.
- Leaving and re-entering the room will change the item pool to the normal Boss Room item pool.

The Fallen can appear on the first floor during a Victory Lap if a Devil Room was entered during the previous lap. Even more rarely, it is possible to encounter him on a normal run if the first level was rerolled with Forget Me Now or a 5-pip Dice Room after visiting the Devil Room by teleporting with a Joker card, a Red Chest, or similar methods.

Trivia

The Fallen appeared in the Halloween Update for the original The Binding of Isaac.
Under normal circumstances, the Fallen is one of six bosses (the other five being Conquest, Headless Horseman, Krampus, Baby Plum if spared, and Bumbino if he picks up enough coins) who drop or can drop active items upon defeat.
The Fallen is both visually and behaviorally similar to Krampus.
Despite the first phase of The Fallen having a death animation before splitting into clones, the clones themselves don't have a death animation. The clones now have a death animation, like the main Fallen.
Previously, only Crusade would play when The Fallen appeared as Satan's first phase. As of Booster Pack #5, Cerebrum Dispersio also has a chance to play in this situation.
- This also applies to when the Angels are fought normally (by bombing the Angel statues).
The Boss might represent Satan or another fallen angel who fell from grace and became evil.
The Boss might have 2 phases to showcase how fallen angels refuse to let go of sin.
The Fallen is tied with Krampus as having the highest Stage HP among all entities in the game.
Double Trouble Fallen encounters in The Womb/Utero do not require the player to have entered a Devil Room before. Upon defeat, they will still drop a Devil Room item.
The Fallen could appear in the alternative chapters in Antibirth.