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.
Lost Fly deals 7 contact damage every 7 ticks, or 30 damage per second. Enemies near Lost Fly will target it instead of Isaac.
Some enemies that do not normally give chase to Isaac, such as Mulligans, will chase after the Lost Fly if nearby it.
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.
The fly will glow slightly in Curse of Darkness, making its position always visible.
The name of the item is likely a reference to the maze-solving algorithm called the left-hand rule, which is based on walking forward and turning left as far as possible until you find an exit to the maze.
Guts.
If any enemies walk into its path, it will deal 7 contact damage per tick or 105 damage per second.
Lost Fly deals 7 contact damage every 7 ticks, or 30 damage per second. Enemies near Lost Fly will target it instead of Isaac.
Mulligans, will chase after the Lost Fly if nearby it.
Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.
Curse of Darkness, making its position always visible.
Fly Items 
The Binding of Isaac: Rebirth
