Ultra Pestilence

Ultra Pestilence is a boss added in The Binding of Isaac: Repentance and is the second of the Ultra Harbingers fought before The Beast in.

Phase 1
Ultra Pestilence spawns with a swarm of Ultra Pestilence Flies that hover around itself for the entire duration of the fight. It has two attacks which it alternates between:
 * Launches a large projectile at Isaac's position. Upon landing in the lava below, it creates a pillar of poisonous cloud, creates a burst of projectiles that shoot upwards from the lava, and creates 3 additional projectiles that shoot upwards from the lava on both sides of the pillar. This attack is repeated 3 times.
 * Fires a large maggot from its eye which flies across the screen, wrapping around three times. Before it completes a fourth wrap, it reverses direction, wrapping around three more times before re-entering Ultra Pestilence's eye. Where the Maggot comes from depends on how many screen wraps it has done:
 * After the first screen wrap, the maggot aims at the top third of the arena.
 * After the second, third, and fourth screen wrap (the maggot reverses directions after the third), it aims at Isaac.
 * After the fifth screen wrap, it aims at the top third of the arena again.
 * After the sixth and final screen wrap, it aims at Ultra Pestilence, re-entering its eye when it hits them.

Phase 2
After dropping to 40% health, Ultra Pestilence loses its head, begins to periodically spawn Ultra Pestilence Flies around itself, and uses two attacks:
 * At the start of this phase, Ultra Pestilence's steed spawns an Ultra Pestilence Fly Ball which follows Isaac. Attacking this ball of flies spawns additional flies. Ultra Pestilence performs this attack one more time after dropping to 20% health.
 * Fires a spray of projectiles from its empty neck, which rain down on the area in front of it.

Trivia

 * Ultra Pestilence, like the other Ultra Harbingers is an upgraded version of its regular counterpart, Pestilence.
 * Upon death, Ultra Pestilence explodes into Small Maggots that rain down harmlessly into the lava.
 * Ultra Pestilence has the least health out of all the Ultra Harbingers, the others have 2000 health while Ultra Pestilence has 1800.