The Pile

The Pile is a boss added in The Binding of Isaac: Repentance that can appear in, as well as in the in the alternate path.

Behavior
The Pile slowly slides around the room slowly and performs the following attacks:
 * Burrows underground, then quickly reemerges elsewhere in the room and fires 5 bone projectiles towards Isaac. The Pile repeats this attack 3 times in a quick succession. It will generally try to reemerge close to Isaac or directly in his path to cut him off.
 * Where it might reemerge can be generally determined by the direction its seen travelling just as it begins to burrow, however this is not always accurate and most likely due to a bug: it can sometimes appear in a location it did not appear to be travelling to.
 * Summons a Bony with a maximum of 2 alive at once.
 * Summons spikes in each of its cardinal directions which start at the edges of the room and move towards the Pile.
 * Charges in Isaac's direction and continues until it rams into a wall or an obstacle, creating two diagonal lines of rock waves away from the wall and launching a spray of bone projectiles in Isaac's direction. If it runs into an obstacle it also destroys it along with any other obstacles in a small radius.
 * The Pile gains dramatically increased knockback resistance while charging.

Trivia

 * In Antibirth, The Pile could appear as a boss in Mausoleum.
 * It appears to be a skeletal variant of Polycephalus and an even further decayed version of The Stain.
 * In its boss art, the rightmost head lacks 2 teeth on the bottom jaw unlike its enemy sprite.
 * The Pile is one of two bosses that can appear in both the main path and the alternate path, the other being Reap Creep.
 * Because of this, its battle theme will be different, depending on where it's fought. If it's fought in Necropolis, either Crusade or Cerebrum Dispersio will play, while if it's fought in the Ashpit, Juggernaut will play.