It's called Merthur, or Marthur, Merlin/Arthur or Arthur/Merlin, and even if I've never seen it called Arlin I guess it's still possible
To me this slash is rather obvious. It's like the authors didn't get that subtext was meant to be
subtle. If you've watched the show once and if you're aware of the existence of the word "Slash", you can't not have thought about it, concerning Merlin and Arthur.
Their relationship is a bit of a cliché: their first encounter is a disaster, they don't like each other from the beginning and,
slowly very quickly, they're ready to risk their lives for the other one.
But even if their story isn't really original, they're just too cute. Physically, you have to fall for Merlin's cheekbones and Arthur's jawline. And the fact that those two have wonderful blue eyes doesn't help when you try to refrain the envy to yell "
eyesex!!" anytime they're in the same room xD
Apart from that, their paths are intertwined, their destinies are linked, they're two sides of the same coin, blahblah, and the Slash Dragon isn't helping you to keep calm.
So, was that a good introduction, y/y?
And since I'm not good at picspamming, check this out