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

I think Arthur's reaction would have been similar 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 had been thinking/doing. Once he realized that Merlin was not going to give up magic, he'd 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 and talks with Gaius/Gwen/the Knights and weighing of options, Arthur would probably come to the realization 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 and Arthur was too weak to move or chase Merlin off himself). Then, after a day or so of isolated travel and just generally lamenting that he'd never known Merlin at all, Arthur began to observe and ask questions and eventually got over it.