Think of the movie "Highlander". Even the title is a little joke about becoming immortal by climbing higher and higher stealing others' life force as you go. And think of The Shining. You have to show you have the belly for the work. Kind of like in the mob you have to "make your bones" to become a made man, but worse because it has to be something you love, a pet or a friend. Proves you're one of them, a creature of pure will.