Deathly Diadem (Patreon)

Wondrous item, very rare (requires attunement)

This gruesome crown was worn by a powerful lich before falling into the hands of the adventurers who slayed it. While wearing this crown, you have resistance to necrotic damage, are always under the effects of the tongues spell, and you can cast the speak with dead spell at will. When you cast speak with dead in this way, you can only ask the target 1 question, to which the target answers with either "yes", "no", or "maybe". If the question can't be answered in this way, the spell ends. In addition, while you wear this crown, undead creatures have disadvantage on attack rolls against you.

Curse. This crown is cursed, a fact that is only revealed to you when you die. The first time that you die while attuned to this crown, its curse is extended to you, and you are immediately returned to life with 1 hit point. While cursed, your alignment becomes evil, if it isn't already, and your utmost goal is to become an all-powerful lich. While cursed, your creature type becomes undead, in addition to your other existing creature types.

A creature remains cursed in this way until targeted by a wish spell or divine intervention. Removing the crown fails to end the curse. While a cursed creature may not reveal that the diadem is cursed, its allies may be able to deduce its presence and effects based on the creature's behavior.

They thought they rid the world of an evil. In reality, they only postponed its next rise to power.