Merthur

Merthur is the slash ship between Merlin and Arthur Pendragon from the BBC Merlin fandom.

Canon
At first, Merlin does not think highly of Arthur, believing him to be a spoiled prince. This is due to their first meeting, where Merlin called out Arthur for bullying another kid; however, this ultimately leads to Merlin being thrown in prison for a day.

Later, Merlin visits The Great Dragon, who has been imprisoned beneath Camelot by Uther Pendragon. From the dragon, Merlin learns of his destiny to assist Arthur in fulfilling his role as the Once and Future King, uniting Albion.

Their friendship begins to grow after Merlin saves Arthur's life from a vengeful witch. Uther, impressed by Merlin's actions, gives him the job of being Arthur's manservant, much to both of their displeasure.

Throughout the series, Merlin protects Arthur from the foes which try to snuff him out before he fulfills his destiny. However, Merlin does much of this in secret since magic is outlawed in Camelot. Due to this, Arthur believes Merlin is rather useless, but he still thinks of him as a good friend. Their bond, for example, if evident in "The Labyrinth of Gedref", where Arthur must pass a trial to save his people from a magical famine and drought. During this test, he and Merlin are sat at a table, on which rests two goblets. One contains poison and the other does not, and one person can only drink from one goblet. Before Merlin can do anything otherwise, Arthur pours both concoctions into one goblet and drinks it all. His willingness to die for Merlin allows Arthur to pass the test.

At the end of the series, Arthur is fatally wounded, and Merlin finally reveals his ability to use magic. Arthur is surprised and somewhat betrayed, but he ultimately accepts Merlin since he used his magic for good. After Arthur dies, Merlin lays him to rest in the Lake of Avalon, awaiting the foretold day when Albion is in need and he will rise once again.

Fanon
On AO3 it is the most written ship for both characters. It is the most written ship in the BBC Merlin tag.

Fandom
AO3

Trivia

 * Although both characters appear in Once Upon A Time, many people do not ship them in that series, due to their limited interactions, and Arthur being evil in the series.