The Oathbreakers

'Thus spoke  Malbeth the  Seer,  in the days of  Arvedui,  last king at Fornost,' said Aragorn:
     Over the land there lies a long shadow,
     westward reaching wings of darkness.
     The Tower trembles; to the tombs of kings
     doom approaches. The Dead awaken;
     for the hour is come for the oathbreakers;
     at the Stone of Erech they shall stand again
     and hear there a horn in the hills ringing.
     Whose shall the horn be? Who shall call them
     from the prey twilight, the forgotten people?
     The heir of him to whom the oath they swore.
     From the North shall he come, need shall drive him:
     he shall pass the Door to the Paths of the Dead.
(...)
 'I  hope  that the  forgotten people  will  not have  forgotten how  to fight,' said Gimli; 'for otherwise I see not why we should trouble them.'
     'That we shall know if ever  we come to Erech,' said  Aragorn. 'But the oath  that  they broke  was  to  fight  against Sauron, and they must  fight therefore,  if  they are to fulfil it. (...)
'Then Isildur said to  their king: "Thou shalt be the last king. And if the West prove mightier than thy  Black Master, this curse  I  lay upon thee and thy folk: to rest never until your oath is  fulfilled. For this war will last through years uncounted, and you  shall be summoned once again ere  the end."
(...)
Aragorn dismounted, and standing by the Stone he cried in a great voice:
     'Oathbreakers, why have ye come?'
     And a voice  was heard  out of the  night that answered him, as if from far away:
     'To fulfil our oath and have peace.'
J.R.R. Tolkien Lord of the Rings - Return of the King