IsaacHDIsaac Rebirth

The Blastocyst (ブラストシスト Burasutoshisuto) is a rare boss composed of a large gelatinous bubble with an Embryo in its centre that will occasionally appear in the Womb/Utero. Rarely, instead of the large form, two medium and two small Blastocysts will appear. Several non-Boss variations with a shell are occasionally found in the Womb (or very rarely in the Depths), though the weakest versions (with just the non-hostile Embryo) can be found in most floors, particularly in rooms with a blood donation machine. Due to its behavior, it is essentially a weaker Monstro version encountered in The Womb.


In its largest form, Blastocyst can do a low hop, sometimes scattering bullets all over the place. It can also do a large jump shooting in 8 directions upon landing. It will split into two smaller versions upon taking enough damage.

In its next form, Blastocyst can shoot a 4 way shot either in the four cardinal directions or diagonally when it lands after jumping. It also splits into 2 smaller forms upon taking enough damage.

In its next form, it fires a 4 way shot in the four cardinal directions after being still for a short time. It does not shoot when landing after jump. It loses its bubble upon taking enough damage and becomes an Embryo.

Champion Variant


Eternal Blastocyst

Eternal Blastocyst will always spilt into 3 smaller versions upon taking enough damage. Its bullet density is also increased.

An additonal Eternal Angelic Baby is spawned when it loses its bubble and becomes Embryo.


  • Also known as "Blobby" or "The Embryo" according to the Unholy Edition art booklet.
  • In mammalian biology, a blastocyst is a hollow fluid-filled cellular structure formed in early pregnancy which contains the cells which will eventually form an embryo.



Regular Bosses
BlastocystChubDaddy Long LegsDark OneDingleDuke of FliesThe FallenFistulaGeminiGurdyGurdy Jr.GurglingsThe HauntLarry Jr.LokiMama GurdyMask of InfamyMega FattyMega MawMonstroMr. FredPeepPinPolycephalusScolexTurdlingsWidow

Afterbirth: BrownieDangleLittle HornRag ManTurdlings
Afterbirth †: Big HornRag Mega
Antibirth-only: Baby PlumBeelzeblubChimeraGreat GideonHornfelThe RainmakerReap CreepRotgutThe ScourgeThe SirenTuff TwinsWormwood

Posthumous Bosses
The AdversaryThe Blighted OvumThe BloatThe CageCarrion QueenThe GateThe HollowThe HuskLokiiMonstro IITeratomaThe Wretched

Afterbirth: The ForsakenThe FrailThe Stain
Antibirth-only: The HereticThe PileThe Visage

Alternative Bosses

Afterbirth †: Sisters Vis

ConquestWarFamineDeathPestilenceHeadless Horseman
Final Bosses
???IsaacIt LivesThe LambMega SatanMomMom's HeartSatan

Afterbirth: HushUltra Greed
Afterbirth †: Delirium
Antibirth-only: The Witness

Seven Deadly Sins
Seven Super Deadly Sins
Super EnvySuper GluttonySuper GreedSuper LustSuper PrideSuper SlothSuper WrathUltra Pride
Angels / Demons
KrampusGabrielUrielFallen Angels
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