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 is also encountered as the first phase of the Satan battle.
 * It cannot appear on the first floor.
 * 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.

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 laser in each cardinal direction.

Phase 2
The Fallen deals a whole heart of contact damage in both phases.
 * 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.

Trivia

 * The Fallen appeared in the Halloween Update for the original The Binding of Isaac.
 * Under normal circumstances, the Fallen is one of five bosses (the other four being Conquest, The Headless Horseman, Krampus, and Baby Plum if spared) 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.
 * Prior to Booster Pack #5, only Crusade would play when The Fallen appeared as Satan's first phase. As of the aforementioned Booster Pack, Cerebrum Dispersio also has a chance to play in this situation.
 * This also applies to when the Angels were fought normally (by bombing the Angel statues).

The Fallen