Board Thread:Questions and Answers/@comment-110.33.239.213-20121226023324/@comment-23.124.224.232-20200709045729

Arthur being the Once and Future King means that Arthur was the king, as you saw, and he will come back when Albion's need is greatest so is also the Future King. Once and Future King.

A lot of people have mentioned how Arthur will come back when Camelot's need or Britain's need is greatest. However, Kilgharrah said that Arthur will come back when Albion's need is greatest. In accordance with the Season 5 Finale, Merlin is all that is left of Albion, so Arthur will come back when Merlin's need is greatest. Arthur will literally come back to life for Merlin.