Demonic Siege Bow

Weapon (any bow), uncommon (requires attunement)
  What this weapon lacks in maneuverability it makes up for in power. Wrought from the horns, bones, and sinew of demons this magic weapon holds malice in every fibre. With its very own bracing mechanism, a user can free up a hand to fire using both strings at once.
  A Strength score of 13 or higher is required to use this weapon. The damage die of this weapon is increased by 1 (i.e. a longbow becomes a d10, a shortbow a d8).
  Siege Brace. As an action, you speak the bow’s command word causing a skeletal, demonic hand to burst from the ground and hold the bow steady until you use an action to dismiss it. The bow can’t be moved while grasped by the skeletal hand; a successful DC 13 Strength check is required to wrest it from this grasp. While in this ‘siege mode’, each time you make a ranged weapon attack using this bow you can make one additional attack against the same target (no action required). When you do so, both attacks are made with disadvantage.