Rex quondam, Rexque Futurus
This Latin phrase was supposedly carved upon Arthur's tomb at Glastonbury, according to Sir Thomas Malory's Le Morte d'Arthur 21:7:
Yet some men say in many parts of England that King Arthur is not dead, but had by the will of our Lord Jesu into another place... many men say that there is written upon his tomb this verse: Hic jacet Arthurus, Rex quondam, Rexque futurus.
Translated in full, the phrase is "Here lies Arthur, King Once, and King in the Future"--or as T.H. White so succinctly translates it, "The Once and Future King."
Of course, the Glastonbury Cross has a different inscription, usually given as: "HIC IACET SEPULTUS INCLITUS REX ARTURIUS IN INSULA AVALONIA":
"Here Lies the Tomb of the Famous King Arthur on the Isle of Avalon."
But see the w/u on that.
As for the return of King Arthur, the messianic element of the Arthurian mythos is not to be overlooked. While a common theme in medieval literature--Charlemagne is also said to sleep under a mountain--the reason for its attachment to King Arthur should be examined.
It is not uncommon for an oppressed people--in this case, the Britons, soon to be Welsh--to have a type of messianic figure in King Arthur, the last great British king, who waits on Avalon and will return in the time of greatest peril. Jesus will have a second coming, and this element of Christianity is most emphasized during times of crisis. Elijah is said to return; a son of Zoroaster will come; Baldr will be resurrected at Ragnarok; Charlemagne is under Chateau Montsegur2, Francis Drake and Drake's Drum; Holger Danske will rise from his repose (either below Kronborg Castle at Elsinore, or Nonnebakken at Odense) and fight to preserve Denmark in her hour of need1; and so on. The idea that a hero/savior will appear one day and drive out the oppressors is a popular one, and understandably so.
However, don't get your hopes up.
1. Thanks, liveforever
, for the addition. I'm willing to add as many messiahs as possible.
2. Ditto legbagede