Blood Caster- An Unkindness to Magicians

Requirement: Spell casting ability score of 15 or higher; The ability to cast 3rd level spells. 

You are able to push your magical abilities beyond your normal limits, at the cost of your own life force. When you have used all your spell slots, you can continue to cast spells of 3rd level or lower. When casting a spell this way, make a DC 15 saving throw using your spell casting ability score. On a success, you suffer force damage equal to 5 x the level of the spell cast. On a fail, you take twice this amount of damage instead. A natural 20 on this save results in taking no damage. You cannot use this feat to cast a spell if doing so would cause your hit points to fall to 0. When using this feat to cast concentration spells, you take the damage before the spell is released, avoiding the need to make an immediate concentration check.