You have mastered the art of converting your own life force into magical power. You may cast spells of 5th level or lower without spending a spell slot. Immediately after a spell is cast this way, roll 1d10 per level of the spell cast, and then reduce your maximum hit points by the amount rolled. If you are reduced to 0 maximum hit points as a results of this reduction, your maximum hit points become 1 and you cannot use this feat again until you finish a long rest (this will still leave you at 0 hp).
After completing a long rest, restore your maximum hit points to their initial value prior to any reductions caused by this feat.