Hemomancy

You have the ability of the long forgotten art of Hemocraft whether it was study in ancient tomes or given by a eldritch entity or a Divine being. You know have access to the hidden potential of your life force. 

This allows you to lower your Max HP to recover spell Slots.

As a bonus action you may recover a spell slot but you must lower you max Hit Points by 5 times the level of the spell slot. This damage is unavoidable and you regain the Hit points when you take a long rest. If you drop your max HP to 0 or below you die outright and if you are revived your max hit points are equal to your total level and you cannot use this ability until you regain all you max hit point.