Unkillable

You have been knocked unconscious and been at the end of your lifespan more times than you can count. You are immune to being Unconscious and when at 0 hp, you can continue fighting. Be warned: If you are hit by the slightest damage while at 0 HP, you die for real. While at 0 hp, you are still half dead. Your movement speed is halved and you have disadvantage on ALL rolls if you attempt to act rather than recover. In order to get it, the DM must feel bad for you. To put a value to it, have 0 HP in 5 separate battles which you participated in.