There is a guy who can't be killed unless he gives the permission to someone to do it. (it's his power). Of course he doesn't want to die so he never gives the permission to anyone.
An assassin that ALWAYS finishes his job is hired to kill the guy. The assassin knows that it's useless to try to kill the guy so he sticks with the guy for his whole life and they become friends.
When the guy is old and would soon die from old age he gives the assassin the permission to kill him. Which assassin honorably executes.