Womb Pillar
Womb Pillars are invulnerable stationary enemies that function as obstacles. When shot at or approached, they will create a large pool of red creep and spray a couple of blood projectiles nearby for a short period. However, they will continue attacking as long as they continue getting shot or if Isaac stays within their vicinity.
They will remain active and will not disappear, even once the room is cleared. If destroyed with an effect such as Euthanasia, they will respawn once Isaac leaves and re-enters the room.
Variants
Utero Pillar
Utero Pillars are variants of Womb Pillars that behave similarly, but will constantly lob 1-2 Organelles nearby, up to a maximum of 6 alive in the room.
Unlike Womb Pillars, they will deactivate once the room is cleared and will disappear after Isaac leaves the room.
Organelle
Organelles are primarily spawned by Utero Pillars, but can appear in some room layouts alone. They will scoot around the room randomly and, on death, create a tiny shower of blood projectiles nearby. They can be killed by walking over them, but unlike Baby Spiders or
Tar Bubbles, they will damage Isaac when doing so.
Organelles do not need to be killed to clear the room.