your immense willpower allows you to keep fighting through the pain. you must roll on a d20 a varying dc check and on a success your willpower overcomes you hp dropping below zero. the first dc is 5 and it scales in fives (5,10,15,20) and if you get hit after all of those checks you must succumb to the boundaries of mortality and go unconscious.  it also allows the use of following spell without spell slots.