Pestilence (ペステレンス,Pesuterensu), one of the four Horsemen of the Apocalypse, is a boss which can be encountered instead of a traditional boss in the Caves and Catacombs once the player has beaten the game for the first time (killed Mom).
He is a bloated, diseased, and zombie-like being with green colors that lazily leans back on his rotting steed as it carries him around.
Pestilence has two main phases:
Like Sloth, he fires a lobbed projectile similar to IPECAC, which explodes on contact, and does one heart of damage only. As Well, Pestilence says "hey". It also leaves a pool of green creep upon exploding.
His movement leaves a large, green creep.
He summons Chargers and Spitties periodically.
After reducing his health to one half, his head is destroyed, leaving his body still mounted on his horse. It still leaves the green puddles of creep wherever it goes and fires the green bombs more rapidly. Also, the horse begins to summon red and black Flies, with the same coughing sound as the Hive (to compensate for the fact that he is no longer able to summon Chargers and Spitties during this phase).
Phase 1 shoots 6 white bullets and summons Trites. Also leaves a white creep (similar to the Widow's) that slows down Isaac if stepped on.
Phase 2 summons a small Spider and shoots 6 blood bullets.
Eternal Pestilence slowly regenerates HP during Phase 1.
Shoots 5 IPECAC shots in a rapid session. All of them leave creep upon explosion.
Once it's in Phase 2, Pestilence's head is detached from his body (appears as Eternal Headless Horseman's Head). Both the body and head leave creep. He does not regenerate HP in Phase 2.
The body summons 2 Red Flies and fires 2 IPECAC shots that leaves creep upon explosion and constantly leaves creep.
The head acts exactly the same as Headless Horseman's Head, but constantly leaves creep.
Pestilence quote often erroneously replaces Conquest in popular culture. The actual text of the Revelation describes Conquest as the first of the four horsemen of the Apocalypse (described as a rider with a bow on a white horse (Rev. 6:2)) and makes no mention of Pestilence.
"A Forgotten Horseman" achievement might be a reference to the fact that Conquest is often forgotten in popular culture.
Pestilence is the only horseman in the game that doesn't grab his horse. (Excluding the Headless Horseman)
Pestilence means a fatal epidemic disease
Occasionally, regular Pestilence will fire three of its shots at once, like a Super Sloth. It can fire as many as 8 of these shots in a rapid succession.
Mostly of the time this happens Pestilence will not show its animation.
Occasionally, as soon as the battle with Pestilence begins, he will fire a single IPECAC shot towards Isaac while he is in the doorway without showing his animation.
Pestilence may glitch into the wall if you push him hard enough, rendering the fight impossible.