Blood Hammer

You have learned how to turn your own pain against your enemies. Magic is in your very blood.

Immediately after taking damage you can use a reaction to make a melee spell attack against the creature who harmed you, shaping and hardening the blood from your wounds into a hammer like tool. On a successful hit the target takes 1d6 magical bludgeoning damage. This damage increases by 1d6 when you hit 5th level, and again at 11th (3d6), and 17th (4d6).

Optional rule changes:

Blood Slash; the damage is changed to magical slashing. Your blood forms a thin blade you slash across the target.

Blood Pierce; the damage is changed to magical piercing. Your blood forms a spear like point to pierce the target.