Looking for a fic about a prince

[coffeeanon]

I don't remember everything, but it was a fairly long fic where Arthur is king and Merlin has been quietly miserable the whole time. I don't think Arthur knows about his magic. Some prince from a kingdom of magic comes along and convinces Merlin to come with him back home, where he can use magic as he likes. I'm not sure, but I think it had something to do with the idea that Arthur wasn't actually the Once and Future King?? Anyways, Merlin agrees, but ends up going back to Arthur in time to help with a battle at the border. The other prince goes with.

Anybody have any ideas? I've been looking for AGES.