Darastrix's Ravenous Horde

The caster drives any number of undead creatures under their control via the Animate Dead, Create Undead, or Finger of Death spell into a frenzy, granting them an extra 10 feet of movement in combat, advantage on attack rolls, and an additional 2d6 necrotic damage on a successful attack action. Due to the strain created by this spell, affected creatures take d4 necrotic damage at the start of their turn as long as the caster maintains concentration (this damage ignores immunity and resistances). This spell can be immediately dismissed as a bonus action.