Mother

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

Behavior
Upon entering the boss room in 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.
 * 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 large Fistula at Isaac that bounces off walls. Mother swipes 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 Fistula chunks outwards. Each medium Fistula flies until it collides with a wall, sending out 2-3 small Fistula chunks and being destroyed in the process. These small Fistulas travel a short distance before bursting and spawning a Small Maggot.
 * Occasionally Mother retreats upwards, becoming invulnerable, and summons a horde of 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 enough HP, Mother retracts her head and arms into her body before releasing a green laser down the middle of the arena, destroying the lower wall and expanding the area of the room. 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 one half of the room, then across the other 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.

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

 * 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.
 * 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, , 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, they can bring Mother to a complete stand still 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.

Trivia

 * As revealed in the Ending that comes after this fight, Mother and Mom are the same person, with Mother being a representation of Mom from Isaac’s perspective.
 * With this in mind, Mother could be considered a posthumous version of Mom.
 * 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", tying it into the Mama Gurdy line and explaining its visual similarities to that line.
 * 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.
 * 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 and Mom's Heart, Mother is not instantly killed by using.
 * Mother is one of the only three bosses that don't have a visible health bar of their own. The others are Mega Satan and The Beast.
 * While upon defeat The Witness turned into stone, Mother instead sinks into the ground. The reason for this is most likely due to the stone corpse of The Witness being able to sometimes soft-lock the player preventing them from reaching the end chest unless they quit out and then continued the run.
 * 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 reaction of to the Fistula attack is likely a reference to the "Dead Man's Volley" commonly seen during boss fights in Zelda.
 * An unused rotten version of Teratoma exists in the files that may have been used in this fight similar to the Fistula ball.