Source: Phandelver and Below - The Shattered Obelisk

Wondrous Item, Very Rare (Requires Attunement)

This soft, violet cap bears stitching in the pattern of folds on a brain.

As a bonus action while wearing the cap, you can project psychic energy in a 60-foot cone. Each creature in that area must make a DC 15 Intelligence saving throw. On a failed save, a creature takes 5d8 psychic damage and has the stunned condition for 1 minute. On a successful save, the creature takes half as much damage only. At the end of each of its turns, a stunned creature can repeat the saving throw, ending the stunned condition on itself on a success.

Once this bonus action is used, it can’t be used again until the next dawn.