The Oubliette

The Oubliette is a black iron sphere 3 inches in diameter, wrapped in chains. It grants its attuned user the ability to cast imprisonment as a Bonus Action, with a save DC of 22. The user can only choose the "hedged prison" option, creating a demiplane in the form of a doorless, windowless cell. Only one creature at a time may be subjected to the spell. The spell ends only if the user's attunement to the artifact is lost, at which time any creature imprisoned by the artifact appears in the nearest empty space adjacent to the artifact.

Attuning the artifact comes with some risk. The creature attempting to attune is subjected to the befuddlement spell with a save DC of 22. If the saving throw fails, or the Psychic damage from a successful saving throw kills the creature, the attunement fails as well.