You have been dead for so long you have become quite difficult to kill.
Benefit(s): When you are reduced to 0 hit points, you are not destroyed— you cannot be killed unless your body is completely destroyed with fire or doused in holy water, or you take damage from a cure spell or channeled positive energy after you’ve been reduced to 0 hit points. Anytime you are reduced to 0 hit points, you remain unconscious until you are healed or until the following midnight, at which time you regain 1 hit point per Hit Die and awaken.
Pathfinder Roleplaying Game Monster Codex © 2014, Paizo Inc.; Authors: Dennis Baker, Jesse Benner, Logan Bonner, Jason Bulmahn, Ross Byers, John Compton, Robert N. Emerson, Jonathan H. Keith, Dale C. McCoy, Jr., Mark Moreland, Tom Phillips, Stephen Radney-MacFarland, Sean K Reynolds, Thomas M. Reid, Patrick Renie, Mark Seifter, Tork Shaw, Neil Spicer, Owen K.C. Stephens, and Russ Taylor.