Ruinstone

A fiery red crystal the size of a human palm.

Once per day as a bonus action, a villain can activate the ruinstone to undo one deed they have performed. The possibilities here are broad, but in combat it's simplest to allow them the opportunity to "rewind" one action and take it again, or to reroll a failed saving throw (if it can still take actions).

Each use of the artifact has a terrible side effect: someone known to the wielder is also erased from reality. After the villain uses the ruinstone, roll a d20: on a 5-20, one of their allies disintegrates, on an 1-4, one of the player characters disintegrates.

If a creature bound its soul to the ruinstone in the "Thalivar's Beacon" quest, the artifact explodes when used and the creature who is bound to it disintegrates, disappearing from reality and can only be brought back with a Wish spell, or similar magic.