'The Dead' was a name for those Men who were 'cursed' by Isildur to haunt the Paths of the Dead after they broke their oath to serve him.

And this they did, for more than three thousand years, until Isildur's rightful heir, Aragorn, traveled the Paths of the Dead during the War of the Ring and called upon them to fight for him at Pelargir.
Encyclopedia entry originally written by Nienna-of-the-Valar