Blastocyst

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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
 
Boss
Stage The Womb icon.pngStage Utero icon.pngStage Scarred Womb icon.png
Stage The Void icon.png
Super Secret Room Icon.png
Degraded Boss
Small
Stage The Basement icon.pngStage The Cellar icon.pngStage Burning Basement icon.png
Stage The Caves icon.pngStage The Catacombs icon.pngStage Flooded Caves icon.png
Stage The Depths icon.pngStage Necropolis icon.pngStage Dank Depths icon.png
Stage The Womb icon.pngStage Utero icon.pngStage Scarred Womb icon.png
Medium
Stage The Womb icon.pngStage Utero icon.pngStage Scarred Womb icon.png
Stage The Chest icon.pngStage Dark Room icon.png
Large
Stage Sheol icon.png
Stage The Chest icon.pngStage Dark Room icon.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 Rebirth 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 The Basement icon.pngStage The Cellar icon.pngStage Burning Basement icon.png
Blighted OvumDingleThe Duke of FliesFamineFistulaGeminiGurglingsThe HauntLarry Jr.MonstroPinStevenWidowThe FallenThe Headless Horseman
Added in Afterbirth DangleLittle HornRag ManTurdlings
Chapter 2
Stage The Caves icon.pngStage The Catacombs icon.pngStage Flooded Caves icon.png
Carrion QueenC.H.A.D.ChubDark OneGurdyGurdy Jr.The HollowThe HuskMega FattyMega MawPeepPestilencePolycephalusThe WretchedThe FallenThe Headless Horseman
Added in Afterbirth The ForsakenThe FrailThe Stain
Added in Afterbirth † Big HornRag Mega
Chapter 3
Stage The Depths icon.pngStage Necropolis icon.pngStage Dank Depths icon.png
The AdversaryThe BloatThe CageThe GateGishLokiMask Of InfamyMonstro IIWarThe FallenThe Headless HorsemanMom
Added in Afterbirth Brownie
Added in Afterbirth † Sisters Vis
Chapter 4
Stage The Womb icon.pngStage Utero icon.pngStage Scarred Womb icon.png
BlastocystThe BloatConquestDaddy Long LegsDeathLokiiMama GurdyMr. FredScolexTeratomaTriachnidThe FallenThe Headless HorsemanMom's HeartIt Lives
Added in Afterbirth † Sisters VisThe Matriarch
Added in Afterbirth Blue Womb
???
Added in Afterbirth Hush
Chapter 5
Stage Sheol icon.pngStage Cathedral icon.png
SatanIsaac
Chapter 6
Stage Dark Room icon.pngStage The Chest icon.png
???The LambMega Satan
Added in Afterbirth † The Void
Stage The Void icon.png
Added in Afterbirth † Delirium
Added in Afterbirth Greed Mode
Ultra Greed
Added in Afterbirth Ultra Greed
Added in Afterbirth † Ultra Greedier
The Binding of Isaac: Rebirth The Binding of Isaac: Rebirth The Binding of Isaac: Rebirth
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