Heart of Damned Intervention (Patreon)

Wondrous item, legendary (requires attunement by a cleric or paladin)

This large infernal heart was torn from a powerful archdevil and is 1 foot tall and weighs 10 pounds. It still pulses, softly, and spews out small gouts of flame from an unquenchable fire within. The heart is hot to the touch and is wrapped by strange, writhing chains.

There is a glimmer of good in this heart that can be used to purify it. While attuned to the heart, you have resistance to fire damage, and you can cast the hellish rebuke spell as a 2nd-level spell at will.

Curse. This heart can only be purified and destroyed by a powerful celestial or deity, and attuning to it overwhelms your heart with its evil desires. The heart is cursed, and attuning to it extends the curse to you. When you attune to the heart, the heart's flames turn pitch black as it painfully sears into your body, taking the place of your own heart, and taints your connection to the gods. You take 15d6 necrotic damage, leaving chain-shaped scars across your torso, and your alignment becomes evil if it wasn't already. While cursed, you and other creatures within 100 feet of you are unable to magically communicate with any god, and any radiant damage you deal becomes necrotic damage instead. In addition, you can understand and speak Infernal and have disadvantage on attack rolls against devils and on saving throws against their spells and special abilities.

If you die while cursed in this way, your soul is bound to the lower planes and can only be restored to life by a wish spell or divine intervention. Only a god or a wish spell cast by a good-aligned creature can end this curse, destroying the heart of damned intervention and restoring your original heart and connection to the gods.

In addition, while you're cursed, you can use an action to speak a devil's true name to summon it, causing it to appear in an unoccupied space you can see within 60 feet of you. The devil must have a challenge rating of 5 or lower. Roll initiative for the devil, which has its own turns. When you summon it and on each of your turns thereafter, you can tell the devil what to do on its next turn (no action required). If you don't issue a command, the devil spends its turn attacking any creature within reach that's attacked it. The devil disappears and returns to its plane of existence after 1 hour or when it reaches 0 hit points. Once this property has been used, it can't be used again until the next dawn.

While cursed, when you use your Channel Divinity, it becomes Channel Damnation instead:

Channel Damnation. As an action, you point your finger at a creature you can see within 120 feet of you and speak a powerful, damning curse. That creature and all creatures within 10 feet of it must make a Charisma saving throw. On a failed save, a creature takes fire damage equal to 1d12 + your cleric or paladin level and is bound by searing, hellish chains. On a successful save, a creature takes half as much fire damage and isn't bound by the chains. A bound creature's speed is halved and takes 1d12 fire damage at the start of each of its turns for up to 1 minute. At the end of each of its turns, the creature can make another Charisma saving throw. On a success, the effect ends on the creature.

The heart yearns for more. It begs for forgiveness, for salvation, for deliverance. I am here to say that all hearts can carry good within them, but some are well beyond the point of redemption.