Kozmotis "Pitch" Pitchiner
Pitch is the Witch King of Angmar.
When he received one of the nine rings, he didn’t fall because of greed but because of love. The life of his daughter was threatened, and to spare her life he agreed to work for Sauron for as long as he lived and into the next live, as long as her life was spared and she was protected.
Because his wrath and power builds on love, he has become the strongest of the nine and became the With King.
As someone pointed out on tumblr; this is why no man can kill him; it has to be a woman; someone who resembles his daughter.