It's been driving me nuts, I've tried dozens of keywords on Google and I still can't find that bloody fanfic, so I will try my luck here.
So the premise of the story (though my memory of it might be faulty, it's been two years? since I read it) is that Uther and Arthur are some sort of anti-magic law enforcers (I think they are part of anti-magic division?) and for some reason they (or Arthur was) are chasing Merlin. I don't know if I'm remembering this correctly or somehow my brain has made it up but Arthur and Merlin have some sort of power over each other. I think they are compelled to obey each other? And there was UST. I know Arthur ends up cornering Merlin to force him to confess that he wants Arthur and they have sex in an antique shop(?). And somehow Uther gains control over Merlin and he orders him to betray Arthur or something. I'm really sorry, I have shoddy memory. Please bear in mind that I might be mixing fanfic plots together. But I do hope what I have written enough to go on. I'll add more details here once I remember them.
Thanks in advance!
Apparently it's Push by corilannam. I wonder why I remember our boys doing it at an antique shop, not the department store's store room. Anyway, I would like to thank the lovely brunettepet for finding this for me.