He looks like a regular demon, with shiny red eyes, grey-colored skin, long horns, ugly-looking teeth, black wings, and a Cross of St. Peter on his forehead. His body ends with a nubby tail section.
The Fallen has 2 phases and total 3 attacks.
The first consists of him chasing after Isaac at high speed. He will stop after taking enough damage and unleash his second attack, launching 4 blood beams (similar to Brimstone) in cardinal cross directions.
For his last attack, he'll spit 2 salvos of bullets, one with 3 and the other with 4. When his health reaches 50%, he will enter his second phase, splitting into 2 smaller Fallens, each with the same attacks as the original.
The Fallen may appear intimidating, but as his patterns are very predictable; he can be killed with relative ease. The projectile waves that he shoots can be dodged easily by weaving in between the bullets or just moving to the side. In phase 2, when there are 2 smaller Fallens, it is best to kite them around the edges of the room during their charge attack.
If Isaac has The High Priestess Tarot Card it is possible to defeat the Fallen in one hit simply by using the card. Mom's foot will kill both the original Fallen and the 2 smaller Fallen immediately as they spawn, finishing the fight almost instantly.
- The Fallen is also part of the first phase of the battle against Satan.
- The Fallen will always drop a Devil Room Item when he is encountered as a boss.
Splits into three of itself at half health, with one of the Fallens appearing slightly larger than the other two.
- The Fallen has many similarities with Isaac's diverse demonic appearances.
- His name is most likely a reference to the fallen angels that followed Satan, in keeping with the Biblical themes of the game.
- From the original game, The Fallen's name text from the "V.S" screen has an inverted cross symbol at the end.
- In the credit sequence, The Fallen is misspelled as "The Fallin."
- According to the game's creator Edmund McMillen, it is possible to find a champion version of The Fallen, but it is extremely rare.
- However, this is actually incorrect, and no champion version of The Fallen exists in the game.
- The Fallen has 2 different facial expressions when spitting shots and chasing from the original game. But in Rebirth while using the spit attack, his facial expression was reused from his chase attack.
- From the original game, while idle, his expression seems to look calm. In Rebirth, his eyes appear to be very stern-looking.
- The Fallen's artwork from the "V.S" screen in Rebirth look very different to the one from the original game.
- If The Hourglass is used while The Fallen is splitting, he will split into 4 instead of 2.
- If Isaac have Dead Bird and it is attacking The Fallen before he splits, when he does split the dead bird will disappear until the next floor.
- Sometimes when fighting The Fallen's 2 smaller parts, they will teleport to opposite ends of the room. This only seems to happen when being encountered in a non-boss room.
- When you use Best Friend while The Fallen is splitting and the explosion hits one of the smaller Fallen while he's damaging Isaac, the game will freeze and he will die.
- If Isaac attack The Fallen right after he splits, it is possible to deal heavy damage.
- Sometimes while The Fallen's about to split into 2 smaller versions, Krampus will appear for a split second few times.