Mega Satan is a final boss appearing in both The Dark Room and The Chest from Chapter 6. He is assumed to be one of the true final bosses appearing at the end of the final chapter, the other being Delirium. Defeating him will instantly unlock the ending (ending 16) of the game and nothing else.
As of the Afterbirth expansion, every character unlocks new co-op babies for beating Mega Satan and it is no longer the final ending of the game. Also, these achievements are Afterbirth exclusive and cannot be earned in Rebirth. (?)
Due to his huge size, Mega Satan's head and a pair of hands are only visible while most of his body are hidden in the darkness. As his name suggests, he appear as a larger, more fearsome version of his original self. His face no longer resembles to a goat's, but little similar to that of a demonic-looking bull. His horns are pointing to the west and east direction, and are much longer and nearly curly like those of a bull or ox. Each hands possess pointy nails. His flaring nostrils constantly snort with smokes. His beard is also thicker than usual. His mouth now sport with visible teeth-gums. His red eyes are dimmed and no longer "shiny"-looking. A cross-shaped mark from his forehead is now X-shaped.
In his second form, Mega Satan's appearance is skeletal with empty eye-sockets, palish brown-grey color, and side-way slits for nostrils. He lacks a beard causing his puffy-looking chin to be clearly visible. Each ears are replaced with a short horn, making Mega Satan to receive four more horns. Furthermore, the color of his horns are changed. A pair of tusk-like fangs are now visible. The X-shaped mark from his forehead is the only remaining feature.
In any later walkthroughs, if the player blows up the rock behind the angel statue in Angel Room by any method, the statue will disappear and either Uriel or Gabriel will appear, forcing the player to battle. If the character have defeated one of them only once, the other angel will be guaranteed to be spawn at the next time the player repeat this action.
Uriel will drop the item "Key Piece #1", while Gabriel drops Key Piece #2; both of which may increase the chance of finding an Angel Room instead of a Devil Room. Collecting both pieces will form a big golden key floating behind the character, similar to a familiar without any function (Both angels can be defeated more than once in a single playthrough, but after the first time, no key piece will be dropped).
On top of that, at the first time the player make a complete golden key, Dad's Key will be unlocked.
Finally, when the character arrive The Dark Room or The Chest, the golden key will fly automatically into the golden door and open it. If the character use the Forget Me Now or being teleported into the I AM ERROR room and then they go down into the trapdoor, the whole floor will be reset and they will waste the golden key. It is possible to fight ??? or The Lamb before entering the Mega Satan room, allowing the defeat for both bosses.
By using the Dad's Key, the character can also open the big golden door.
Upon entering the room, his reversed "ISAAC" line is heard while the pentagram from the floor moves to the upside-down pattern. Afterwards, Mega Satan appears with his laughter playing.
The battle between Mega Satan includes 2 main phases, while the first one includes 3 "mini-phases".
1st FormIn this phase, both Mega Satan's head and hands are vulnerable; they don`t share their hit points though. If one of his hands are killed, after a period of time it will respawn.
- If Mega Satan is vulnerable, he keeps using these attacks without any certain regular pattern:
- He shoots out several waves of blood shots, covering all directions.
- He shoots out five straight lines of blood that curve to the right, and alternate to the left, like the Wiggle Worm.
- He shoots out many blood shots that form a flower- like pattern.
- He shoots out 6 ~ 7 waves of blood shots (3 x 3 = 9 shots in a row), covering all directions but still leaving small space between rows.
- He shoots multiple brown bullets. Each one splits into four smaller bullets when they hit the ground (similar to the boss version of Isaac`s attack).
- He slams down one of his fists, making a rocky shockwave that moves outward, similar to Adversary`s headbutt attack.
- He rapidly slams down both of his fists alternately, making several waves of shockwaves in front of them. The width of these shockwaves increase at the beginning, and decrease after they reach the middle of the room. There is no reaction time or clear indication before he uses this attack.
- He shoots a giant Brimstone beam out of his mouth straight ahead. The beam will cover about one third of the screen and lasts for a long time.
- He headbutts the floor for several times, spreading disorganized blood bullets. Similar to Monstro`s spreading attack but covers all directions and shoots much more bullets.
- After taking enough damage for the first time (the certain number is unknown since Mega Satan doesn't have a health bar), Mega Satan will retreat into the darkness, becoming invincible. He'll summon Famine and Pestilence; after they are defeated, War and Death will be summoned. Defeating all these 4 Harbringers will force Mega Satan to go back. Whether these knights are champion varieties is depended together rather than separately.
- After taking enough damage for the second time, Mega Satan retreats again. This time, he'll summon all the super counterparts of Seven Deadly Sins in a certain order: Super Envy on its own, followed by Super Lust and Super Wrath, and finally Super Gluttony, Super Sloth, Super Greed, and Super Pride together.
- After taking enough damage for the third time, Mega Satan retreats once more and summon total 2 Fallen Angels. First, he summon a black version of Uriel; defeating it will cause Mega Satan to summon a black version of Gabriel.
- Note that if too much time was spent during the battle, Mega Satan may randomly summon 2 of the normal Seven Deadly Sins.
- Rarely, Mega Satan can summon Larry Jr. or The Hollow with about 7 segments.
2nd FormAfter taking even more damage, he slowly sinks into the darkness, seemingly defeated; then he rises up again in his skeleton form while this slower version of his laughter playing. In this phase, he does not summon enemies anymore and hands disappear, but shoots a bullet hell.
- The skeletal Mega Satan keeps using attack 1, 2, 3, 4, 5 that are mentioned above without any certain regular pattern, but he loses the other four attack forms and gain new attack forms:
- He shoots out more than 5 rows of flames that have varying color.These flames are just a kind of bullets in fact, therefore can be blocked by Orbitals or Trinity Shield.
- He shoots an enormous number of red and black bullets that cover almost all the screen, moving in different patterns.
- He shoots rings of red and black bullets that moves in a Wiggle Worm pattern.
- He shoots multiple spinning black bullets.
- He shoots metallic gray bullets that moves in a Wiggle Worm pattern.
- He shoots metallic gray bullets which pulses out (move for a little moment, stop, then repeat).
When he takes damage during this phase, his skull begins to crack; After defeated, his skull collapses. Mega Satan does not drop the Golden Chest after his defeat, instead the player is forced directly to Ending 16.
As the very final boss of Rebirth, Mega Satan is the most difficult one among all bosses, even harder than The Lamb and ???. Without enough attributes and items, it's almost impossible to defeat him; therefore, it is strongly recommended to clear all the rooms in The Dark Room or The Chest before fighting Mega Satan since multiple items might be gained.
In the first phase, the wide brimstone beam can be a great threat; therefore, comparing to stay right in front of him and shooting, it is recommended to defeat one of his hand first and shoot him at the side of his head`s hitting box. His fist smash can be easily dodged while his bullets are not that many (comparing to the second phase).
When he summons the 4 Horsemen, it is better to kill Pestilence and Death earlier in each pair, because the summoned monsters or green Creep left by Pestilence`s movement and IPECAC shot can damage Isaac (unless he have an item with a flight ability), and Death`s scythe attack is hard to dodge when he summons the Super Sins, especially the third wave of 4 Sins. The room may be messed up and usage of AOE damage items is suggested.
In the second phase, anything that blocks projectiles are very useful such as Orbitals, Trinity Shield, and Infamy (especially Trinity Shield since it can block all bullets from ahead). If Isaac do not have any of these items, he have to move carefully into the small spaces between the bullets; the best way to accomplish this is by focusing solely on Isaac and his immediate surroundings - avoiding any distraction caused by the patterns the projectiles create.
- He is immune to Midas' Touch, unlike other bosses.
- From Isaac's Last Will after getting killed by Mega Satan's second form, his skull will display.
- He shares the same battle theme as the regular Satan, which is titled "Hericide".
- Mega Satan is one of the bosses to receive the word "Mega" for his name. Two others are Mega Maw and Mega Fatty.
- Mega Satan's appearance could bear some resemblances to Mortemor's third form, the final boss from Dragon Quest VI.
- The four horns of Mega Satan's second form look strongly similar to the Jacob sheep, a rare sheep breed said to have from two to six horns, but most commonly have four.
- Out of all bosses, Mega Satan is the only boss so far to lack a health bar which makes it hard to determine how close he is to death.
- From the game's files, three sprites of Mega Satan's bony-looking hands for his second form can be seen from one of his sprite sheets. They were meant to be used if his hands weren't destroyed during his second form. The skeletal hands, however, weren't used in the game - even if Mega Satan's hands still weren't killed after beating his first form, they instead disappear.
As in the Afterbirth expansion, the achievements when defeating Mega Satan are now exclusive. (?)
Cry Baby - Defeat Mega Satan with Isaac.
Red Baby - Defeat Mega Satan with Magdalene.
Green Baby - Defeat Mega Satan with Cain.
Brown Baby - Defeat Mega Satan with Judas.
Blue Baby - Defeat Mega Satan with ???.
Lil' Baby - Defeat Mega Satan with Eve.
Rage Baby - Defeat Mega Satan with Samson.
Black Baby - Defeat Mega Satan with Azazel.
Long Baby - Defeat Mega Satan with Lazarus.
Yellow Baby - Defeat Mega Satan as Eden.
White Baby - Defeat Mega Satan as The Lost.
Big Baby - Defeat Mega Satan as Lilith.
Noose Baby - Defeat Mega Satan as Keeper.
Mega Blast - Defeat Mega Satan as every character.