The Witness

The Witness is a new final boss introduced in Antibirth, always found at the end of the Corpse.

When entering the boss room in Corpse II, the battle will not start. Instead, the room contains four golden chests, all of which offer items (similar to The Chest's starting room), and a large pit in its center. Walking into the pit will cause the player character to fall through and start the final battle.

Upon beating The Witness, a knife completion mark will be added in an extra post-it note.

Behavior
The Witness has two phases. In its first phase, it is attached to the upper wall and has several attacks:
 * Fires 3 lines of red tears that follow Isaac which burst upon hitting a wall.
 * Shoots 2 parallel lasers from its right eye after it spawns rocks in random places for Isaac to stay behind
 * Smashes one of its fists and spawns rock waves that travel outwards.
 * This movement is only used to erase rocks formed by the sweeping laser attack.
 * Smashes one of its fists and spawns a ring of 10 tears that split.
 * Spawns multiple Corpse Eaters from its right hand.
 * Spits a cluster of blood shots with 3 larger tears that stick in the ground, leaving behind red creep for a few seconds and periodically shoot 6 shots.
 * These tears can be destroyed and will continue to fire shots until they are destroyed.
 * Throws a massive knife leaving a trail of bullets. Bullets explode from the location the knife impacts the wall, and the trail of bullets splits horizontally in both directions.
 * Spits a ball of blood that will bounce off of the wall and back to it. It will then smack the ball back at Isaac. The ball will bounce between The Witness and the far wall several times, increasing its speed and size each time it bounces.
 * Smashes its fists to spawn moving rows of tears along with rows of tears that travel in lightning patterns.

Once the first phase's health is depleted, it will detach from the wall and begin roaming around the room with new attacks:
 * Spawns tears at an edge of the room, which all then create lines of tears across the room after a short delay.
 * Charges at Isaac, leaving behind tears that explode sideways upon hitting a wall or impassable terrain, identical to The Parasite item.
 * Fires a large tear that then transforms into a rotating brimstone beam. It will always make space for Isaac to avoid when it stops in the wall or corner.
 * During this attack, The Witness will simultaneously fire lines of bullets in 4 directions, rotating counterclockwise until the beam vanishes.
 * Spins around in place, simultaneously firing 5 rotating rows of bullets and rings of 20 bullets. Each ring alternates between rotating clockwise and counterclockwise.
 * Fires a cluster of bullets at Isaac that slightly homes and splits into 6 smaller versions upon contact with a wall. The smaller versions home in as well and burst into a random cluster of bullets upon hitting a wall or after a certain amount of time.

Unlockables

 * Jacob & Esau - Defeat The Witness for the first time.
 * Seeing Double - Defeat The Witness for the first time.
 * Isaac's Awakening - Defeat The Witness for the first time.
 * Meat Cleaver - Defeat The Witness as Isaac.
 * Yuck Heart - Defeat The Witness as Magdalene.
 * Red Key - Defeat The Witness as Cain.
 * Akeldama - Defeat The Witness as Judas.
 * Eternal D6 - Defeat The Witness as ???.
 * Bird Cage - Defeat The Witness as Eve.
 * Donkey Jawbone - Defeat The Witness as Samson.
 * Devil's Crown - Defeat The Witness as Azazel.
 * Tinytoma - Defeat The Witness as Lazarus.
 * 'M - Defeat The Witness as Eden.
 * Lost Soul - Defeat The Witness as The Lost.
 * Revelation (currently bugged, unlocks nothing) - Defeat The Witness as Bethany.
 * Magic Skin - Defeat The Witness as Jacob & Esau.
 * Mega Mush - Defeat The Witness as every character on hard mode.

Trivia

 * The track played in The Witness battle is titled Memento Mori. Memento Mori means "remember that you have to die" in Latin.
 * The name may be a reference to a murder victim, as you killed your own mother's heart much earlier to unlock the boss, and it could have possibly been watching.
 * In the New Testament Book of Revelation, two figures referred to as "the witnesses" were sent to preach on Earth before the "end of days".
 * The Witness originated as NotYourSagittarius' mockups of Ur-Gurdy in January 2015, a rumoured boss originating from a vandalism on the wiki's Bosses page, later claimed by a Reddit user.  It is the final form of all Gurdies. Its design is similar to a gigantic form of Gurdy that would act as a boss in Sheol.
 * The Boss's Phase 2 was orignally supposed to be Phase 3. However, Phase 2 got scrapped.
 * The Witness is the post-mortem version of Mama Gurdy.
 * The Witness was officially incorporated to the official The Binding Of Isaac: Rebirth game (along with most of Antibirth 's content) in the Repentance DLC, where it is renamed to Mother.