Eternal Fly is a type of fly that was added to the Wrath of the Lamb. They're pure white in color.
They can be found on any floor in the game. They usually circle Corn Poop or enemies like Holy Leeches and the Hanger and will not stop circling them until their hosts are destroyed. Before that, they will deal damage to Isaac if he touches them and are completely immune to all forms of damage. When their hosts are destroyed, they convert into large Attack Flies with increased health.
If Eternal Flies are circling Corn Poop and there are no other enemies around, the character can leave the room at will. This is true even if they have already been converted into large Attack Flies: they will simply disappear when the player returns into the room. This method will only work in Basement / Cellar.
If the character has an enemy-seeking Familiar such as Dead Bird or stage 3 or 4 Cube of Meat, they will often preoccupy themselves by attempting to kill initially-invincible Eternal Flies, leaving Isaac to fight other enemies unaided. The player should destroy their Corn Poop to make the fly vulnerable, so the Familiar can kill it and move on.
- If the host of an Eternal Fly is destroyed while it is frozen due to the use of items such as Mom's Bra, the fly will simply sit in the same spot without doing anything, being pushed by any tears shot at it.