User blog comment:Death'sapprentice77/The Witch King Vs. Algol (God Match?)/@comment-422690-20100919020314/@comment-181289-20100919021527

But the Witch-king ring keeps his soul to the world, that's why he lived for so long eventully to the point his phyiscal form faded. It's not just his his standing as a wraith that makes him hard to kill, but also magic defences palced on him by Sauron.