Immediately grants 3 broken hearts. Every otherwise fatal hit will instead add 2 more, replacing other types of hearts if necessary, and deal 40 damage to all enemies in the room.
Once Isaac has no non-broken hearts left, he dies. Extra lives will apply at this point.
Found Soul: The ghost gains 2 extra hits per floor and every hit they take will damage all enemies in a room as well.
Keeper: Keeper will lose all coin hearts and gain 3 broken hearts, resulting in immediate death without extra lives.
The Lost: The Lost has full benefit of the item, but the amount of broken hearts is invisible. This lets him survive 4 extra hits, each hit adding 1 damage, and the 5th is fatal.
Tainted Jacob: Tainted Jacob's Lost form has its own health bar, although invisible. If Heartbreak is collected in normal form, the Lost form can take 5 extra hits, the 6th is fatal; collected in his Lost form, he has 4 extra hits. The broken hearts are retained between levels, and are not shared between his forms, i.e. hits taken in either form will not count against the other.