Spawns a familiar which spawns friendly blue flies in a room with monsters when Isaac holds down a fire key.
The initial blue fly will target a monster once nearby, in which Rotten Baby will lock its blue flies automatically onto the same one, immediately heading towards and damaging it until it dies.
This item belongs to the Conjoined set. Collecting three items from this set will transform Isaac into a three-faced version of himself.
Notes
This item can rarely drop from destroying polyps in Corpse.
Spawns one Fly at a time.
Blue Flies deal 2x Isaac's damage.
Flies will be generated regardless of Isaac's ability to fire tears normally.
Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with Rotten Baby having the lowest priority at 6th / 8th and thus being farther from Isaac than familiars of different priorities.
Synergies
Fish Tail: Rotten Baby will spawn two flies at a time.
The Twins: Unlimited flies can be slowly generated by walking back and forth between rooms while firing. A new fly will generate each time Rotten Baby is duplicated.
This item belongs to the 
Conjoined set. Collecting three items from this set will transform Isaac into a three-faced version of himself.
This item can rarely drop from destroying polyps in 
Corpse.
6th / 
8th and thus being farther from Isaac than familiars of different priorities.
Fish Tail: Rotten Baby will spawn two flies at a time.
Mongo Baby: Mongo Baby will also spawn flies.
BFFS!: Rotten Baby becomes larger, but this is only a cosmetic change.
Hive Mind: Fly damage is doubled.
The Twins: Unlimited flies can be slowly generated by walking back and forth between rooms while firing. A new fly will generate each time Rotten Baby is duplicated.
Bug!
