|The Chest or The Dark Room|
| N/A (Cuts directly to Ending 16)|
|Defeating ??? or The Lamb|
Mega Satan is a unlockable boss in The Binding of Isaac: Rebirth.
- In subsequent runs, Angel Statues can be bombed to fight an Angel boss. When killed, they will drop either Key Piece 1 or Key Piece 2.
- Collecting both key pieces will form the fully completed key, which is automatically used upon entering The Chest or the Dark Room on the golden gate.
- Alternatively, Dad's Key can be used to unlock the Golden Key Door in lieu of the key pieces. Dad's Key is unlocked in the basement by collecting both key pieces in an earlier run.
Passing through this Golden Key door and stepping onto the pentagram will initiate the Mega Satan fight.
- Mega Satan has several attacks:
- He can spit out a fast 3 or 4-directional spread of blood shots, with little to no space between consecutive shots.
- He can fire a wide Brimstone beam directly downwards.
- He can repeatedly smash his head into the platform, sending scattered blood shots in all directions.
- With one hand, he can smash the platform, sending a circular wave of rocks outwards.
- He can rapidly smash both hands in front of him, sending waves of rocks forwards.
- After a certain threshold of damage, he will summon all four Horsemen of the Apocalypse in pairs, Famine with Pestilence followed by Death with War. His face and hands will be invincible while he withdraws to watch the fight.
- After another threshold, he will summon all seven super sins in three waves: Super Envy on its own, followed by Super Lust with Super Wrath, and finally Super Gluttony, Super Sloth, Super Greed, and Super Pride together.
- After a third threshold, he will summon a dark version of the white-robed Angel boss found in the Angel Room. After it is killed, a red-robed version of the dark Angel will appear.
- Taking too much time between thresholds will cause Mega Satan to start summoning normal sins.
- After taking even more damage, Satan will sink into the darkness and rise up in his final phase as a skull. His final form does not spawn any other enemies; instead, he fires complex, bullet hell-style shots. Possible patterns include:
- Streams of red shots
- Rings and streams of multi-colored fire
- Streams of simultaneous red and black shots, which move in different patterns
- Brown shots which split into 4 smaller brown shots, similar to Cricket's Body
- Rings of black shots that pulse outwards
- Streams of black shots that move outwards in a Ring Worm pattern
- Streams of metallic gray shots which move outwards in a zigzag pattern
- Rings of metallic gray shots that move in a Wiggle Worm pattern
- As Mega Satan takes damage during this phase, his skull will begin to crack. After this final phase is defeated, the game ends and the final ending cutscene plays.
- Currently in the game there is no reason to battle Mega Satan other than to get the last ending, as he doesn't unlock anything after being defeated.
- Mega Satan does not have a health bar, thus sometimes making it difficult for the player to determine how close they are to killing him.
- Mega Satan does not drop a Golden Chester after beating him, instead cutting directly to Ending 16.
- Doing too much damage between phases will cause the game to play the normal "boss defeated" music throughout the rest of the fight.
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