Board Thread:What If?/@comment-39182132-20190510012113/@comment-37017073-20190511232130

I think Arthur would have reacted similarly to how he reacted to Gwen's betrayal in 4x09. He'd freak out and have Merlin arrested (magic is against the law, after all), then have a private conversation once he'd calmed down to find out what the heck Merlin was playing at. Once he understood the situation and realized that Merlin wouldn't be giving up magic, he'd probably decide that he couldn't trust him (or that he needed time to think without worrying about whether he was being enchanted) and banish him/send him home to Ealdor. Then, after a lot of soul searching, weighing of options, and talks with Gwen, Gaius, and the Knights, Arthur would probably realize that he could trust and/or forgive Merlin after all.

Which is pretty much how the actual reveal went in 5x13. Arthur did freak out and he did beg Gaius to send Merlin away the first chance he got (banishment wasn't really an option since they were alone in the middle of nowhere, plus Arthur was too weak to move and therefore couldn't chase Merlin off himself). Then, after a day or so of isolated travel and generally lamenting that he never knew Merlin at all, Arthur began to observe and ask questions and eventually came to understand that Merlin was the same person he'd always been.