As a reaction, if you would normally be reduced to 0HP by an attack, instead be reduced to 1HP and enter Determined Stance. Upon entering Determined Stance, all of your class specific resources (action surges, ki points, etc) are restored to their maximum. While in Determined Stance, all attacks that you make do their maximum amount of damage. You also cannot drop below 1HP while in Determined Stance. Determined Stance lasts for 3 turns, and at the end of your third turn you can choose to extend your Determined Stance by a max of 3 turns by making a CON save (DC:15). When exiting Determined Stance, you can either use the Last Resort feature or take all of the damage you absorbed while in Determined Stance.
Last Resort: When exiting Determined Stance, you take the damage you have sustained this combat (can go over your max HP if you've been healed) along with the total damage you have dealt during Determined Stance, then deal that damage to one target. This attack ignores resistances and immunities, but will instantly reduce you to 0HP, and if you decided to extend Determined Stance by any additional turns, the number of turns that you extended by will affect how severely your body is injured. For example, extending by one turn will cause your body to sustain extreme damage, requiring 8th level healing spells to restore to regular functionality, extending by two turns will cause your body to sustain even more extreme damage, requiring 9th level healing spells to restore, and extending by three turns will cause your body to sustain damage that cannot be healed by any means.