The darkness will not swallow you until you have finished what you set out to do.

Daily   ✦     Primal
Immediate Interrupt      Personal

Trigger: You drop to 0 hit points or fewer and don't die

Effect: You are dying but don't fall unconscious because of that condition. At the end of your next turn, you fall unconscious if you are still dying.

Published in Player's Handbook 2, page(s) 55.