Aragorn-King of the Dead/Paths of the Dead

[lathspell]

'Hear now the words of the Heir of Isildur! Your oath if fulfilled. Go back and trouble not the valleys again! Depart and be at rest.'
And thereupon the King of the Dead stood out before the host and broke his spear and cast it down. Then he bowed low and turned away; and swiftly the whole grey host drew on and vanished like a mist that is driven back by a sudden wind.

This is the part which tells the answer (which Orodhromeus had already given) to your question.

5th&6th page of The Last Debate - RotK - LotR