With Bone Hearts, Red Hearts contained within the Bone Hearts will be taken before regular Red Heart damage, but the Bone Hearts will not be fully depleted.
If Isaac only has half a Red Heart left, Soul and Black Heart damage will be taken, and Bone Hearts can be destroyed.
This is useful for getting into Boss Challenge Rooms when you want to maintain your Soul/Black Hearts but have more than one filled Red Heart container.
Red Heart damage taken via the trinket affects your Devil/Angel Room chances normally.
As a result, this trinket makes Soul/Black Hearts useless at protecting Devil/Angel Room chances.
This is useful when playing as The Forgotten, as it will preserve his Bone Hearts (and Devil/Angel Room chance) until all of his Bone Hearts are empty.
This trinket does nothing while controlling The Soul.
This is useful for getting into Boss Challenge Rooms when you want to maintain your Soul/Black Hearts but have more than one filled Red Heart container.
Eve Unlocks 
The Binding of Isaac: Rebirth 
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