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.

Behavior
The Witness has two phases. In its first phase, it is attached to the upper wall and has several attacks: Once the first phase's health is depleted, it will detach from the wall and begin roaming around the room with new attacks:
 * Fires 3 lines of red tears that follow Isaac which burst when 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 move 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.
 * Fires a cluster of bullets at Isaac that slightly homes and splits into 5 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.
 * Spits a cluster of blood shots with 3 larger tears that stick in the ground 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 the player. The ball will bounce between The Witness and the far wall several times, increasing its speed each time it bounces.
 * Smashes its fists to spawn moving rows of tears along with rows of tears that travel in lightning patterns.
 * Spawns tears at an edge of the room, which then create lines of tears across the room after a short delay
 * Charges at Isaac, leaving behind portals that attract tears
 * At low health, it fires a large tear that then transforms into a rotating brimstone beam
 * Spins around in place, simultaneously firing 5 rotating rows of bullets and rings of 20 bullets. Each ring alternates between rotating clockwise and counterclockwise.

Trivia

 * 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.
 * The design of the Witness is a possible nod to a rumor that began close to Rebirth's release, the Ur-Gurdy, a gigantic form of Gurdy that would act as a boss in Sheol.
 * The Witness might also be a dead or upgraded version of Mama Gurdy, as they have many similarities.