A permanent that is activated when your health reaches zero, then heals you somehow. Either it just sets your health to a certain amount or it could convert all of your quanta of a specific type into health. Those are just possible ways it could work. It would also have to stop the rest of your opponent's creatures from attacking that turn, or it would be pointless (assuming there was enough damage to still kill you). It can be easily countered via permanent control, and if quanta was converted into health, quanta denial. Not sure what it should do if you have more than one on the field at a time.
And it is destroyed when the ability activates.