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Boss Blastocyst name.png
Boss versus screen image
Table dividing line 1.png
In-game appearance
Table dividing line 2.png
Stage womb.pngStage utero.pngStage scarred womb.png
Stage the void.png
Super Secret Room Icon.png
Stage basement.pngStage cellar.pngStage burning basement.png
Stage caves.pngStage catacombs.pngStage flooded caves.png
Stage depths.pngStage necropolis.pngStage dank depths.png
Stage womb.pngStage utero.pngStage scarred womb.png
Stage womb.pngStage utero.pngStage scarred womb.png
Stage chest.pngStage dark room.png
Stage sheol.png
Stage chest.pngStage dark room.png
Table dividing line 3.png
Boss Boss

Blastocyst is a boss that spawns in all environments of Chapter 4. It is a large blob with a small Embryo inside.

Behavior[edit | edit source]

  • Hops around the room randomly and shoots 8 tears in all directions.
  • Performs a giant leap to the player's position, shooting 12 tears in all directions upon landing.
  • Once enough health is depleted, it will split into two medium versions of itself, which shoots 6 tears in all directions.
    • The medium blastocysts have 75 HP each.
  • These two medium blastocysts each split into two smaller forms, which shoot 4 tears in the cardinal directions.
    • Small blastocysts have 30 HP each.
  • When all the parts of the blastocyst are defeated, they will become a bouncing Embryo that does 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 TechnologyTechnology Icon.pngReplaces tears with a piercing laser with unlimited range. or Brimstone BrimstoneBrimstone Icon.pngTears are replaced by a laser beam that pierces through all enemies and obstacles in its path and deals high damage. Can only be fired after a short charge-up period. can defeat this boss extremely quickly.

Gallery[edit | edit source]

Notes[edit | edit source]

  • Blastocysts are seen in a group consisting either of 1 large Blastocyst and 2 small Blastocysts, 1 large Blastocyst and 4 Embryos or 2 medium Blastocysts and 2 small Blastocysts.

Trivia[edit | edit source]

  • Blastocyst appeared in the original The Binding of Isaac.
  • An embryo is called a blastocyst when the differentiation of cells within a morula finally begins, usually 4–5 days after the fertilization of an embryo.
  • 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. This can also apply if a very large amount of damage is dealt very quickly to any of the phases.
    • Removed in Afterbirth † This can also happen if Blastocyst is killed while still having a debuff that actively harms it (e.g. poison, fire).
  • 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.
  • Added in Afterbirth † During the DLC's release, an unfair room with a Portal in the middle existed in all environments of Chapter 4 consisting of 8 Blastocysts as a result of unintentionally placing full sized ones rather than small ones. It was acknowledged by Edmund McMillen on Twitter and was fixed shortly after.
  • As Blastocyst divides, its positive facial expressions will become less happy until it becomes a bouncing embryo which has no smile at all.

Bosses Boss Monstro.png
Chapter 1
Stage basement.png
Stage cellar.png
Stage burning basement.png
Blighted OvumDingleThe Duke of FliesFamineFistulaGeminiGurglingsThe HauntLarry Jr.MonstroPinStevenWidow
Added in Afterbirth DangleLittle HornRag ManTurdlings
Chapter 2
Stage caves.png
Stage catacombs.png
Stage flooded caves.png
Carrion QueenC.H.A.D.ChubDark OneGurdyGurdy Jr.The HollowThe HuskMega FattyMega MawPeepPestilencePolycephalusThe Wretched
Added in Afterbirth The ForsakenThe FrailThe Stain
Added in Afterbirth † Big HornRag Mega
Chapter 3
Stage depths.png
Stage necropolis.png
Stage dank depths.png
The AdversaryThe BloatThe CageThe GateGishLokiMask Of InfamyMonstro IIWarMom
Added in Afterbirth Brownie
Added in Afterbirth † Sisters Vis
Chapter 4
Stage womb.png
Stage utero.png
Stage scarred womb.png
BlastocystThe BloatConquestDaddy Long LegsDeathLokiiMama GurdyMr. FredScolexTeratomaTriachnidMom's HeartIt Lives
Added in Afterbirth † Sisters VisThe Matriarch
Added in Afterbirth Blue Womb
Added in Afterbirth Hush
Chapter 5
Stage sheol.png
Stage cathedral.png
Chapter 6
Stage dark room.png
Stage chest.png
???The LambMega Satan
Added in Afterbirth Greed Mode
Ultra Greed
Added in Afterbirth Ultra Greed
Added in Afterbirth † Ultra Greedier
Added in Afterbirth † The Void
Stage the void.png
Added in Afterbirth † Delirium
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