Demented Cultist’s Scimitar

Weapon (scimitar), rare (requires attunement)
  This obsidian blade refracts light with an opalescent sheen. The hilt is made of dead driftwood tied together with ragged fabric. You gain a +1 bonus to attack and damage rolls made with this magic weapon.
  Ritual Branding. As an action, you can place a brand on a willing creature, choosing from the list below. The brand lasts until the following dawn, or until dispelled. Once you have branded a creature, you can’t do so again until the following dawn.
  • Brand of the Flagellated. When the branded creature hits a creature with an attack, it can expend a number of hit dice up to its proficiency bonus, dealing an additional 1d6 damage of the attack’s type per hit die expended.
  • Brand of the Devout. As a bonus action, the branded creature can expend a number of hit dice up to its proficiency bonus (minimum 2). It regains one spell slot with a level equal to the number of hit dice expended minus one.
  • Brand of the Madman. As a bonus action, the branded creature can expend a number of hit dice up to its proficiency bonus. Until the start of its next turn, it gains advantage on a number of attacks equal to the number of hit dice expended.
  Very rare variant. Increase the bonus to attack and damage rolls to + 3.