A creature that has a heart in its own body can attune to the withered heart. When it does so, the withered heart switches places with the attuned creature’s living heart. The creature’s living heart then withers and dies, and it gains the same properties as the withered heart, allowing it to be passed on in the same way.
The withered heart doesn't beat and breaking attunement to it leaves the creature without a heart, killing it.

When attuned to the withered heart, you gain the following benefits:

Your creature type is replaced by Undead and you have resistance to necrotic damage.
You have advantage on saving throws against disease and being poisoned, and you have resistance to poison damage.
You have advantage on death saving throws.
You don’t need to eat, drink, or breathe.
You don’t need to sleep, and magic can’t put you to sleep. You can finish a long rest in 4 hours if you spend those hours in an inactive, motionless state, during which you retain consciousness.

Notes: Resistance: Necrotic, Advantage: Saving Throws, Resistance: Poison, Advantage: Death Saving Throws, Immunity: Magical Sleep, creature with a heart in its own body