I think the fact that they broke their oath and Isildur condemned them for doing so was enough, and Isildur didn't have any special powers. It's a common theme in fantasy. One breaks an oath - one lives forever as a ghost, unless he fulfills his oath.
(I suppose this is what you meant)
__________________
"Out of doubt, out of dark to the day's rising I came singing in the sun, sword unsheathing. To hope's end I rode and to heart's breaking: Now for wrath, now for ruin and a red nightfall!"
~Eomer
|