Items with this tag are familiars which are unique and cannot be duplicated by Box of Friends, and Soul of Lilith can grant them only once. Note that this does not prevent getting multiple copies of the items through duplication effects such as Diplopia.
Isaac himself is now immune to all damage, and a heart familiar follows him. If the heart is hurt, Isaac is hurt. The heart charges up as Isaac shoots, firing 8 blood shots, knocking back enemies, and leaving a puddle of creep when released.
Spawns a glass familiar that spills on the floor upon taking damage. After the milk spills, Isaac's tear delay is decreased by 2 for the current room. After the milk spills, Isaac's fire rate is increased by 1 for the current room.
Box of Friends, and 
Soul of Lilith can grant them only once. Note that this does not prevent getting multiple copies of the items through duplication effects such as 
Diplopia.
Key Piece 2, unlocks the golden gate in the starting room of 
Chest and 
Dark Room to fight 
Mega Satan.
Key Piece 1, unlocks the golden gate in the starting room of 
After the milk spills, Isaac's tear delay is decreased by 2 for the current room. 
Red Hearts. Increases damage and speed for the floor based on the amount drained.
Knife Piece 2, creates a knife familiar that damages enemies and can open the door to the 
Corpse.
Knife Piece 1, creates a knife familiar that damages enemies and can open the door to the 
Boss Room. Defeating it drops an extra item.
The Binding of Isaac: Rebirth
