Black Crystal Tablet

From Waterdeep: Dungeon of the Mad Mage, page 284.

Description
Wondrous item, legendary (requires attunement by a creature that has proficiency in the Arcana skill).

Any creature that attunes to the tablet must make a DC 20 Wisdom saving throw at the end of its next long rest. On a failed save, the creature becomes afflicted with a random form of long-term madness.

As an action, a creature attuned to the Black Crystal Tablet can use it to cast Eyebite or Gate (the portal created by this spell links to the Far Realm only). After the tablet is used to cast a spell, it cannot be used again until the next dawn.