Boss Blastocyst.png
Found In
Dropped Items
Boss Pool
Unlocked By

Blastocyst is a boss that spawns in the Womb and Utero. It is a large blob with a small fetus inside. It moves by hopping about similarly to Monstro. Its main source of damage is shooting large tears in all cardinal directions.

Behavior[edit | edit source]

The boss when encountered will simply hop about and shoot tears in all direction on occasion.

Once defeated, it will split into two smaller versions of itself. These smaller versions will shoot tears in 6 directions. These two smaller versions also split into two more even smaller versions of the boss. These now tiny versions shoot weaker tears in 4 directions. Upon defeat they become simply bouncing fetuses that do contact damage.

Blastocyst is perhaps the least complicated boss in this chapter, having essentially one attack that gets weaker for each split. Any sort of piercing tears or items such as Technology 2 or Brimstone can defeat this boss extremely quickly.

Trivia[edit | edit source]

It is very common for the second level of the boss to spawn as normal enemies in rooms. On even later floors like Sheol, the full boss can even spawn as a common enemy.

If Blastocyst is killed on spikes, none of the smaller versions will spawn.

Upon killing a blastocyst, it seems as though, for a split second, an audio clip of someone's voice can be heard. To better hear it, listen with headphones without the music.


Bosses Boss Monstro.png
Basement MonstroGeminiDingleLarry Jr.The Duke of FliesGurglingsFistulaStevenFamine
Cellar Gurdy Jr.PinWidowBlighted OvumThe HauntDingleLarry Jr.The Duke of FliesGurglingsFistulaFamine
The Caves ChubGurdyGurdy Jr.Mega MawMega FattyDark OneC.H.A.D.Pestilence
Catacombs The HollowThe HuskPeepPolycephalusThe Carrion QueenThe WretchedDark OnePestilence
The Depths The CageMonstro IIThe AdversaryThe GateLokiGishWarMom
Necropolis The BloatThe Carrion QueenMask of InfamyThe WretchedThe GateLokiWarMom
The Womb Mr. FredScolexTeratomaMama GurdyThe BloatLokiiDeathConquestMom's HeartIt Lives
Utero BlastocystDaddy Long LegsTeratomaMama GurdyThe BloatLokiiTriachnidDeathConquestMom's HeartIt Lives
Sheol The FallenSatan
Dark Room The LambMega Satan
Cathedral Isaac
The Chest ???Mega Satan
Devil Room Krampus
Angel Room Angel
Levels 1-5/7 The FallenThe Headless Horseman
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