Mother

This article is about the final boss of Corpse. For the boss of chapter 3 and chapter 3.5, see Mom.

Mother is a boss added in The Binding of Isaac: Repentance and the only boss of the second floor of the Corpse, as well as the final boss of Ending 21.

Behavior

Upon entering the boss room in Corpse II, the room is empty except for a hole in its center. Walking into it drops Isaac down into the arena where Mother is fought.

Phase 1

Mother begins the fight attached to the top of the room and is stationary, similar to Mega Satan. Her face and hands can all be attacked to deal damage to her, and she uses them separately in her attacks:

One of Mother's hands grabs a knife and throws it directly downwards. The knife emits projectiles horizontally while travelling and a burst of many projectile lines in a radius upon impacting the back wall. This attack has two variations in terms of the projectiles' paths.
One of Mother's arms releases trails of projectiles that move across the arena in a wave pattern, eventually homing in on Isaac.
One of Mother's hands slams the ground, releasing streams of projectiles in a radius and spawning Small Maggots.
- Small Maggots that land on top of Mother's hitbox do not spawn.
Occasionally, Mother spits out a unique Fistula at Isaac that bounces off walls. Mother swipes the Fistula towards Isaac whenever it travels close to one of her hands and releases 2 lines of 8 projectiles when it collides with the bottom wall. Mother repeats this several times, causing the Fistula to gradually gain speed, before slamming a hand down on the mass, crushing it and sending 3 medium Fistulas outwards. Each medium Fistula flies until it collides with a wall, sending out 2-3 small Fistulas and being destroyed in the process. These small Fistulas travel a short distance before bursting and spawning a Small Maggot.
- This Fistula can be reflected back towards Mother through the use of a variety of melee-based attacks, each reflect causing the Fistula to glow brighter. Successfully reflecting the Fistula 6 times will cause it to be destroyed upon contact with Mother, dealing damage to and briefly stunning her. It will also burst into 8 medium Fistulas in a radius that disappear quickly, six of which deal damage to Mother; each of these can also deal damage to Isaac. Abilities that can cause this interaction include:
  - Spirit Sword's spin attack.
  - The Forgotten/Soul of the Forgotten's bone swing.
  - Tainted Samson/Soul of Samson/Berserk!'s donkey jawbone swing.
  - Bag of Crafting's swipe.
  - Tainted Magdalene's hug attack, though it cannot be triggered by the Fistula.
Occasionally, Mother retreats upwards, becoming invulnerable, and summons a horde of 34 Dead Isaacs at the bottom of the arena that walk up towards Mother. If any are left alive to reach Mother, she swipes with a hand, killing any that are caught in the swipe, releasing a tiny burst of red projectiles, and launching their corpses towards Isaac. After a while, she ends this attack by slamming down both hands, which causes a small rock wave to appear beneath any remaining Dead Isaacs, killing them, and launching their corpses up into the air.

Phase 2

After losing 2222 HP, Mother retracts her head and arms into her body before releasing a green Mega Blast laser down the middle of the arena, destroying the lower wall and expanding the area of the room to be 1x2. She then separates from the top wall, launching her head into the center of the room. From this point on, she slides slowly towards Isaac when she isn't performing one of her attacks:

Spins around in place, shooting out strings of wavy projectiles in all directions.
Inhales and draws Isaac to her as projectiles appear behind Isaac as he moves. After that, she shoots a massive ball of projectiles towards Isaac that splits into many smaller clusters when hitting a wall, which each then send out lines of projectiles when they collide with a wall or Mother.
Leaps upwards off-screen before charging vertically across the half of the room which Isaac is located within, then across the other half in the opposite direction. The half she charges across first is telegraphed by a splat at the top or bottom of the room. After this, she slams down in the middle of the room, sending rock waves and small strings of projectiles in all directions.
Faint white lines appear at set intervals along a random arena wall before a line of projectiles quickly shoots out of the wall along each line; these projectiles hover for a moment before dropping. One line of projectiles is always lined up with Isaac's position. This attack is repeated five times and follows a pattern: vertical lines, then horizontal, vertical, horizontal, horizontal (or vice versa).

Gallery

Mother in-game.
Mother in her second phase.
Fistula's appearance in Mother's attack.
Mother's full splash art.
Mother's old splash art in Antibirth from when she was called "The Witness".
Concept art of an older version of Mother back when she was called "The Witness" for Antibirth. Notably, it has a scrapped 2nd phase that neither made it in the Antibirth mod nor the Repentance DLC.
Later concept art of Mother (earlier known as "The Witness") transitioning from phase 1 to phase 2. This transition was adapted into both the Antibirth mod and the Repentance DLC.
Boss screen mockup of the "Ur-Gurdy", a design which would later be adapted for "The Witness" and Mother.
In-game mockup of "Ur-Gurdy's" fight.

Unlockable Achievements

A Strange Door - Defeat Mother.

Jacob and Esau - Defeat Mother.

Seeing Double - Defeat Mother.

Isaac's Awakening - Defeat Mother.

Meat Cleaver - Defeat Mother as Isaac.

Yuck Heart - Defeat Mother as Magdalene.

Guppy's Eye - Defeat Mother as Cain.

Akeldama - Defeat Mother as Judas.

Eternal D6 - Defeat Mother as ???.

Bird Cage - Defeat Mother as Eve.

Bloody Gust - Defeat Mother as Samson.

Devil's Crown - Defeat Mother as Azazel.

Tinytoma - Defeat Mother as Lazarus.

'M - Defeat Mother as Eden.

Lost Soul - Defeat Mother as The Lost.

Blood Puppy - Defeat Mother as Lilith.

Keeper's Sack - Defeat Mother as Keeper.

Lil Portal - Defeat Mother as Apollyon.

Bone Spurs - Defeat Mother as The Forgotten.

Revelation - Defeat Mother as Bethany.

Magic Skin - Defeat Mother as Jacob and Esau.

Dice Bag - Defeat Mother as Tainted Isaac.

Mother's Kiss - Defeat Mother as Tainted Magdalene.

Lucky Sack - Defeat Mother as Tainted Cain.

Number Magnet - Defeat Mother as Tainted Judas.

Ring Cap - Defeat Mother as Tainted ???.

Lil Clot - Defeat Mother as Tainted Eve.

Swallowed M80 - Defeat Mother as Tainted Samson.

Azazel's Stump - Defeat Mother as Tainted Azazel.

Torn Card - Defeat Mother as Tainted Lazarus.

Modeling Clay - Defeat Mother as Tainted Eden.

Crystal Key - Defeat Mother as Tainted Lost.

Adoption Papers - Defeat Mother as Tainted Lilith.

Cursed Penny - Defeat Mother as Tainted Keeper.

Apollyon's Best Friend - Defeat Mother as Tainted Apollyon.

Hollow Heart - Defeat Mother as Tainted Forgotten.

Beth's Essence - Defeat Mother as Tainted Bethany.

Found Soul - Defeat Mother as Tainted Jacob.

Godhead - Earn all 5 / 9 / 10 / 12 Completion Marks on Hard mode as The Lost.

Damage Scaling

Damage scaling is an unclear mechanic introduced by Nicalis in The Binding of Isaac: Afterbirth to provide a more challenging experience to the players, enabling some enemies to retaliate against high damage attacks by deploying an "armor" of a sort to greatly reduce the amount of damage taken. Further details and an in-depth explanation may be found here.

Strategies

Chaos Card works on Mother, but only kills one of her phases. If you are holding a Chaos Card, it is recommended to use it in Phase 2, since Mother's attacks are much more dangerous.
Curse of the Tower can be very useful, since Troll Bombs will attempt to spawn near Mother, and bombs in Repentance will deal 100 damage and ignore the armor. Around 43 bombs would be able to kill her, which corresponds to about 6~7 times of damage. This will be affordable if you have more than 10 hearts in total.
- Similarly, using Anarchist Cookbook, XVI - The Tower, or your own Bombs will also help.
Her charging attack in Phase 2 can be greatly stalled by placing a bomb in front of her path, while also dealing damage to her in the process.
In phase 2, if the player has Gnawed Leaf, they can bring Mother to a complete standstill while she's charging if they enter statue form in front of her.
- Pair this with at least one offensive familiar and the fight becomes a joke for all characters.
Using Ace of Hearts during the Dead Isaac attack will spawn 10 hearts on average, which should keep any red heart characters alive for quite some time. Using it during the hand slam attack that spawns Small Maggots will usually spawn 3-5.
Using Eraser on Mother's first phase allows phase 2 to be skipped as long she has 15 health or less.
With proper timing, Soul of Judas can one-shot Mother's second phase even with base damage. The best attack for this is the spinning attack, but it is also possible to do this right after Mother spits out the ball of projectiles. Be aware that the boss can dodge the damage by performing the charge attack.

Notes

Using Plan C will kill Mother in time if you have an extra life. Even though Isaac dies well before the animation is complete, the game over screen will pop up almost immediately after the animation ends. If Isaac has an extra life item that revives him outside the room, he will be able to enter the Final Boss Chest without fighting Mother again.
- Killing Mother in this way will not reward the completion mark for the current character if they don't have an extra life.
- Killing Mother in this way will not work for The Lost / Tainted Lost if the revival transports them to the previous room since their death animations are shorter than those of normal characters.
Rotten Tomato or any items that inflict Charm can cause Mother's wavy projectiles during the first phase and second phase to target herself.
The ball of splitting projectiles in Mother's second phase can also collide with Mother. The player can therefore move to the opposite side of Mother after she fires to more easily dodge the attack.
Unlike most other bosses, Mother has no visible health bar in either of her phases, so it is impossible to tell how much damage you are dealing to her without Spider Mod.
Suplex! works on Mother, but in Mother's first phase, Isaac can pick up only one of her parts at a time, either her head or one of her arms, and give it a suplex.
II - The High Priestess will damage Mother even though it can be considered another form of Mom.
Using Black Hole will not stop Mother's wavy projectiles from proceeding once the black hole has diminished, causing an un-dodgeable attack. Using this item during the fight is highly ill-advised.

Trivia

The Ending following this fight reveals Mother to be a grotesque, undead representation of Mom imagined by Isaac before the events of the game.
This boss originates from the Antibirth mod, and was originally named "The Witness".
- The Witness itself was first designed as a boss mockup by the name of "Ur-Gurdy". The Ur-Gurdy was a speculative posthumous version of Mama Gurdy which was rumored to rarely replace Satan in Sheol.
Between her original appearance in Antibirth and Repentance, Mother’s boss fight was changed drastically, including the arena being shrunken dramatically, the replacement of several attacks with new ones, the boss theme being changed, and the boss's death animation being changed, along with the addition of Mom’s voice lines during the battle.
- Because of how she died in Antibirth (turning to stone), it was possible to softlock the game by killing The Witness where the ending chest spawned.
- In Antibirth, the Corpse II boss room contained four golden chests with items (similar to the starting room in Chest). In Repentance, the boss room is completely empty except for the hole leading to the fight against Mother.
Mother's boss theme is called "Morticide". "Morti-" stems from Latin "Mors" which means "death" or "corpse", while "-cide" means "an act of killing", so the track name effectively means "Killing the dead".
The back wall of the fight is actually part of Mother's model; attempting to summon her via the console will change the wall of the current floor to a Corpse wall.
Unlike Mom, Mom's Heart, and It Lives, Mother is not instantly killed by using The Bible.
Mother is one of only three bosses that don't have a visible health bar of their own. The others are Mega Satan and The Beast.
Faint translucent Chubber projectiles can be seen leading the lines of projectiles in Mother's second phase white line attack.
In Antibirth, The Witness was immune to Status Effects.
The interaction of Spirit Sword to the Fistula attack is likely a reference to the "Dead Man's Volley" commonly seen during boss fights in The Legend of Zelda.
According to _Kilburn, the mega Brimstone laser when starting phase 2 is a reference to The Nuclear Throne, a boss from the game Nuclear Throne.^[1]
Mother beating up Dead Isaacs in one of her attacks could be alluding to how Isaac gets beaten up in life after leaving his room.
Mother spawning 34 Dead Isaacs might be a reference to there being 34 playable characters.

Bugs

Bug!	If Mother is killed using the "Quick Kill" debug flag, the final boss chest will not spawn, soft-locking the game.

Bug!	Broken Modem may crash the game during the Mother fight at a random time, usually related to the wrist burst attack. The attack does not always cause a crash.

Bug!	When entering the corpse for the first time: the item unlock screen for defeating Mother as the current character may appear, but the item nor the secret screen won't be unlocked.