Bosses

Always the boss of the Chest. Often called Blue Baby.

No unlock of its own. The guides below show how to reach it.

Used in: Angels, The D20, Celtic Cross, Sacrificial Dagger, Blood Penny, Cain's Eye and 33 more guides

Sources:??? (Boss)

Main boss of The Void, in one of its several boss rooms.

No unlock of its own. The guides below show how to reach it.

Used in: D infinity, Eucharist, Silver Dollar, Shade, King Baby, Bloody Crown and 30 more guides

Sources:DeliriumThe Void

First boss of Home, fought after sleeping in Mom's bed.

No unlock of its own. The guides below show how to reach it.

Used in: Options?, Candy Heart, A Pound of Flesh, Redemption, Montezuma's Revenge, Cracked Orb and 28 more guides

Sources:Home

Unlocked by Blue Womb

Used in: A Secret Exit, Soul of Cain, New Area, Fart Baby, Purity, D12 and 67 more guides

Sources:Hush

Boss of the Cathedral.

No unlock of its own. The guides below show how to reach it.

Used in: The Polaroid, Angels, Apollyon, I RULE!, BRAINS!, Backasswards and 92 more guides

Sources:Isaac (Boss)

Unlocked by It Lives!

Sources:It Lives

Can appear as the boss of a Chapter 4 floor (Womb, Utero, Scarred Womb).

Unlocked by The Womb

Used in: Loki's Horns

Sources:Lokii

Behind the Golden Gate in the starting room of the Chest and the Dark Room.

Unlocked by Angels

Used in: Apollyon, I RULE!, Backasswards, Ultra Hard, Cry Baby, Red Baby and 32 more guides

Sources:Mega Satan

Boss of Depths/Necropolis/Dank Depths II, and of Mausoleum/Gehenna II on the alternate path.

No unlock of its own. The guides below show how to reach it.

Used in: The Womb, A Strange Door, Red Key, Soul of Cain, Tainted Isaac, Tainted Magdalene and 132 more guides

Sources:Mom

Boss of Womb/Utero II until It Lives! is unlocked.

No unlock of its own. The guides below show how to reach it.

Used in: Blue Womb, It Lives!, A Strange Door, ???, Eden, Jacob and Esau and 69 more guides

Sources:Womb

Always the boss of Corpse II.

No unlock of its own. The guides below show how to reach it.

Used in: A Strange Door, Jacob and Esau, Isaac's Awakening, Seeing Double, Meat Cleaver, Yuck Heart and 32 more guides

Sources:MotherCorpse

Boss of Sheol.

No unlock of its own. The guides below show how to reach it.

Used in: The Negative, The gate is open!, Angels, Judas, Apollyon, The Forgotten and 97 more guides

Sources:Satan

Final boss of Home, after Dogma and the four Ultra Harbingers.

No unlock of its own. The guides below show how to reach it.

Used in: Options?, Candy Heart, A Pound of Flesh, Redemption, Montezuma's Revenge, Cracked Orb and 28 more guides

Sources:The BeastHome

Always the boss of the Dark Room.

No unlock of its own. The guides below show how to reach it.

Used in: The gate is open!, Angels, Missing Poster, Gulp! (pill), Demon Baby, Abel and 37 more guides

Sources:The Lamb

Final boss of Greed Mode, on the seventh floor. In Greedier he has a second phase (Ultra Greedier).

No unlock of its own. The guides below show how to reach it.

Used in: Lilith, Keeper, Lucky Pennies, Special Hanging Shopkeepers, Wooden Nickel, Cain holds Paperclip and 57 more guides

Sources:Greed Mode