Lost Fly is a passive item added in The Binding of Isaac: Afterbirth expansion.
- Spawns a fly familiar that moves in a straight line and attaches itself to the first wall or obstacle it comes into contact within each room.
- The Lost Fly will then move around the room functionally similar to the Guts.
- If any enemies walk into its path, it will deal 7 contact damage per tick or 105 damage per second.
- If the obstacle it is attached to is destroyed then it will continue rolling in the direction it was previously traveling until it meets another wall or obstacle.
- This item belongs to the Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.