The Harbingers

The Harbingers is the achievement that unlocks Famine, War, Pestilence, and Death as four additional bosses. The Harbingers are referenced in the Bible as the Four Horsemen of the Apocalypse. These bosses are only unlocked once Mom has been defeated for the first time. These bosses, with the exception of Conquest and Headless Horseman, are guaranteed to drop either a Ball of Bandages or a Cube of Meat. By default, there is a 10% chance the current floor's boss will be a Harbinger unless the floor has a fixed boss (for example, Mom in Depths and Necropolis) or the Harbinger has been fought already. By using the Book of Revelations on a floor, it is guaranteed that the boss will be a Harbinger.

Famine

Famine is the Harbinger for Chapter 1.

Pestilence

Pestilence is the Harbinger for Chapter 2.

War

War is the Harbinger for Chapter 3. War cannot be found in Depths II, Necropolis II, or Dank Depths II as Mom will always be the boss of that floor.

Death

Death is the Harbinger for Chapter 4. Death cannot be found in Womb II, Utero II, or Scarred Womb II as Mom's Heart or It Lives will always be the boss of that floor.

Conquest

Conquest is an alternative boss to Death in Chapter 4. He is unlocked once 10 items have been taken from Angel Rooms. Conquest does not drop normal Horseman items, instead dropping White Pony.

Headless Horseman

Headless Horseman can appear in the place of any Harbinger. Like Conquest, he doesn't drop normal Horseman items, instead dropping A Pony.

Unlockable Achievements

The Book of Revelations - Defeat a Harbinger.

7 Seals - Defeat all 5 Harbingers.

Gallery

Concept art.

Trivia

Conquest was added in the Binding of Isaac: Wrath of the Lamb. In the Bible, Conquest is actually the first Horseman to appear, while Pestilence is a Horseman existing only in popular culture, coming from a different translation of the Greek word 'Thanatos', often meaning Death instead (the canon four Horsemen, thus, are Conquest, War, Famine and Death).
- Conquest's entry in the secrets is called "The Forgotten Horseman", because of that very reason.
Death is the only horseman specifically given a name in The Book of Revelation.
The use of the Book of Revelations item to summon a horseman for the current floor is reference to the fact that the book of the Bible in which the horsemen are described is Revelation.
None of the Harbingers have a death animation.
Under normal circumstances, Conquest and Headless Horseman are 2 of only 5 bosses (the others being Krampus, The Fallen, and Baby Plum if spared) who drop or can drop active items upon defeat.
- Bumbino can drop active items if paid 12 coins, but this does not defeat him in the traditional sense.
- As for abnormal circumstances, the use of Filigree Feather allows the Angels to drop active items, and the use of Satanic Bible allows nearly any boss to drop an active item.
The horses used by the Harbingers resemble hobbyhorses.