Mega Satan is an unlockable boss in The Binding of Isaac: Rebirth.
- ??? or The Lamb must have been defeated in an earlier run.
- 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 by collecting both key pieces in an earlier run.
- Get out of Jail Free Card can also unlock the Golden Key Door if used in the starting room of The Chest or the Dark Room.
- Mr. ME! can also unlock the door if the item is aimed at it.
- Stepping on spikes in a Sacrifice Room for the 9th and 11th times spawn Uriel and Gabriel, who will drop their respective key pieces. Stepping on spikes for the 12th time and beyond has a 50% chance to teleport Isaac to the Dark Room, where the door will be unlocked if both key pieces were picked up.
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 brown tears which split into 4 smaller brown tears, similar to Cricket's Body.
- 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 Uriel. After it is killed, a dark version of Gabriel will appear. They are identical to their normal counterparts, with slightly more health.
- Occasionally, between thresholds, Mega Satan will start summoning normal sins and The Hollow.
After taking even more damage, Mega 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
- The same brown shot attack from the first phase.
- 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 Ending 16 plays.
If The Void is unlocked, there is a 50% chance that the game will not immediately end, and Mega Satan will instead drop an ending chest along with a portal. Entering the chest ends the game and plays the cutscene, while entering the portal will take Isaac to The Void.
These achievements are exclusive to Afterbirth.
- Afterbirth added Completion Marks for defeating Mega Satan. In Rebirth, there are no achievements linked to Mega Satan.
- Mega Satan does not have a health bar, making it difficult to determine how close he is to death.
- Mega Satan does not drop a Golden Chest after beating him, instead cutting directly to Ending 16.
- It will drop a Golden chest if a portal to The Void appears, giving a choice of getting the ending or going to the true final floor.
- Unlike normal bosses, he is immune to Midas' Touch.
- Killing one of his hands deals 100 damage to Mega Satan.
- With homing tears and The Ludovico Technique, the tear will be constantly attracted by the head even when it's untargetable, making killing the horsemen, sins, and angels very hard.
- Unlocking the Golden Gate with the combined key pieces and then restarting the floor with Forget Me Now or an I AM ERROR room will waste the key, which will not respawn.
- Mega Satan's battle theme is titled Hericide, just like regular Satan.
- The Mega Satan room appears square, but is actually a 1x2 room with a large pit where Mega Satan stands.
||Blighted Ovum • Dingle • The Duke of Flies • Famine • Fistula • Gemini • Gurglings • The Haunt • Larry Jr. • Monstro • Pin • Steven • Widow |
Dangle • Little Horn • Rag Man • Turdlings
||Carrion Queen • C.H.A.D. • Chub • Dark One • Gurdy • Gurdy Jr. • The Hollow • The Husk • Mega Fatty • Mega Maw • Peep • Pestilence • Polycephalus • The Wretched |
The Forsaken • The Frail • The Stain
Big Horn • Rag Mega
||The Adversary • The Bloat • The Cage • The Gate • Gish • Loki • Mask Of Infamy • Monstro II • War • Mom |
||Blastocyst • The Bloat • Conquest • Daddy Long Legs • Death • Lokii • Mama Gurdy • Mr. Fred • Scolex • Teratoma • Triachnid • Mom's Heart • It Lives |
Sisters Vis • The Matriarch
| Blue Womb
||Satan • Isaac|
||??? • The Lamb • Mega Satan|
| Greed Mode
|| Ultra Greed |
| The Void
|Chapters 1-4||The Fallen • The Headless Horseman|
|The Binding of Isaac: Rebirth|
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