Originally Posted by
pendell
The magic in this case, IMO, was the oath the people who were now dead swore. An oath, in middle earth, is magic anyone can work.
The living men, then, were the ones who bound themselves to this fate when they swore the oath to Isildur. And since the oath was sworn to Isildur only Isildur or his heir could release them from it. Any king of Isildur's lineage could have done so, but none ever did. And thus they lingered after death in an awful half-life until they could be freed from the power of their own words.