Crimson Lash

As part of casting this spell, you must expend a hit die or the spell automatically fails. If you do, a writhing lash of coagulated blood springs from your hand. This magic weapon lasts until the spell ends. It counts as a simple weapon with which you are proficient, and it has the light, finesse, and reach properties. You can choose to add your spellcasting modifier, instead of your Strength or Dexterity modifier, to attack and damage rolls made with this weapon. Its damage die is equivalent to the hit die expended casting this spell, and the damage dealt is necrotic.  Each time a creature takes damage from this magic weapon, its maximum hit points are reduced by an amount equal to the damage took.