Ouroboros
If Merlin ever dies in battle (and Arthur is beginning to have serious doubts about this outcome, for while his manservant is absurdly clumsy, he also possesses extraordinary good luck), it will be by sword. Arthur knows this to be a fact. Merlin can fight with a sword, just enough to be competent, but he holds a blade as if it is a snake waiting to turn on him, and eyes all bladed weapons warily.
(Arthur mentions this once, and watches as Merlin's face turns thoughtful, and is not quite sure whether or not he hears his friend mutter something about the properties of 'cold iron')
If Merlin ever dies at all, he will die saving Arthur.
(Arthur tries to avoid the vague panic that steals over him when Merlin unnecessarily braves fire and steel for him yet again, because it is obvious that Merlin fears swords, and that forged iron, cold iron, as he calls it, is abnormally lethal to him.)
Merlin's courage will never cease to impress Arthur. The raven-haired man may fear the sword, but he has something like steel, or something stronger, at his core.
(Excalibur rests on Merlin's collarbone, winking. Merlin simply looks at Arthur. The inscription on the blade says 'Cast Me Away', and when his prince asks how many times magic has been used to save his life, the warlock chuckles.)
If an enemy ever conquers Arthur Pendragon, it will not be while Merlin is around.
(An injured Arthur can see the storm in Merlin's eyes, the lightning and dragonfire burning in his soul, and wonders if his enemies accounted for an angry sorcerer in their plans.)
If they ever get old (another outcome that Arthur doubts, for they are too busy to have the time to get old), they will never change.
(A fair-haired king and a raven-haired warlock chase each other around the castle, shouting good-natured insults)
They will face the end of their world as brothers, clinging to a strange, shared knowledge that it is not the end, but the beginning.
(An old man sleeps in a crystal cave, dreaming of the future. His silver hair turns sable over the years, and his veins thrum with magic)
(A fair-haired king sleeps on an island of dreams. His fingers curl around the hilt of a sword with new strength, and the inscription on the blade reads 'Take Me Up'.)
Author's Note: Does anyone have any better ideas for a title?