Source: Bigby Presents - Glory of the Giants

Wondrous Item, Rare (Requires Attunement)

This jagged iron circlet bears ornaments in the shape of the enemy rune. When worn, the crown glows with pale light as it draws upon the wearer’s fury to strike opponents with vicious terror.

When you make an attack roll against a creature and hit it while wearing this crown, you can spend and roll one of your unspent Hit Dice. The creature takes extra psychic damage equal to the number rolled.

Invoking the Rune. As an action, you can invoke the crown’s rune to cast the Fear spell (save DC 15) with it; the spell has a duration of 1 minute and doesn’t require concentration. Once the rune has been invoked, it can’t be invoked again until the next dawn.