Neither John nor Sherlock could blame him for being a little traumatized. They decided to let him rest for the night, maybe try to help him adjust in the morning.
John told Merlin, "You can take the room upstairs if you want. It will be more comfortable than the floor or a chair."
"I've slept on floors before," Merlin argued. "And besides, Arthur might wake up disoriented and need someone to be there for him."
Unexpectedly, Sherlock spoke up. "There's another room just down here you could use." He gestured towards his own bedroom.
John's eyebrows nearly rose into his hair as he looked over at his friend, and after a second, he inclined his head in a silent thank you. Sherlock just gave him a look that said he had no idea what he was talking about. John retaliated with a warm smile that said that yes he bloody well did, but even if he couldn't admit it, John was still grateful to him for arranging things so that Merlin could be closer to Arthur, and that it was very thoughtful of him. The detective finally looked away in embarrassment, and said, "I just need to move some things," and hurried into his room to do so.
The next day, Arthur was still looking traumatized. But at last Merlin managed to convince him to sit up and eat something, and let John (who came back after work) change his bandages, and even to take a shower (which nearly scared him stiff until Sherlock explained how it worked). Once he was dressed in some of Sherlock's spare clothes-a gray button-down shirt and jeans that he used for when he was undercover-he sat back on the couch, with an ice pack on his shoulder, and brooded.
Finally, out of the blue he asked, "How do you know about us? I remember that earlier you and the healer both claimed to remember our lives. Has our legacy really lasted that long?"
"...In a way," Merlin cut in before the detective could answer. He produced a book seemingly out of nowhere, one of the children's stories about the Knights of the Round Table. "This is what people read and believe nowadays."
Arthur took it, and settled down to read.
Before long, he was spluttering indignantly, "That's not how it happened!" or "I never did that!" at least once every two pages. Just as the other three men were ready to kill him if he said either of them or an equivalent one more time, he suddenly jerked up.
"They think that Mordred was-my son?!"
Merlin couldn't help snorting with laughter. Not too long after, the others joined in. They couldn't help it, what with the look on Arthur's face and all. He looked as utterly horrified as it is possible for a man to get at the notion.
Arthur scowled. "It is not funny, Merlin."
The warlock just chuckled harder, though he tried to stop. "Sorry," he managed to get out as he finally composed himself. However, as Arthur went back to his book he smiled at the doctor and the detective so they could see he wasn't that sorry. Arthur, however, evidently saw it out of the corner of his eye, because he lobbed one of the sofa cushions at Merlin. Just before it could make impact, it froze in midair, and then dropped to the floor.
"Hey, no fair using magic!"
"Now that I don't have to keep it a secret anymore, it is completely fair! You're physically the stronger one, so I need to have some compensation!"
"I am the-"
Arthur stopped, remembering that he was no longer king. With a small sigh, he went back to his book.
When he finished, he put it aside. "Well, at least they basically got the ending right." Then he looked over at Merlin. "Why do you think that I came back now? It doesn't seem like there's any way that Camelot could possibly need me now. It's become this-England, and from what he tells me-" he pointed at John- "they already have a monarchy. What use could they possibly have for me?"
Merlin stood up. "I actually have a theory about that. I think the reason why you've been brought back-" he reached into his pants pocket, pulling out a folded newspaper clipping- "is because of this." He unfurled it, and they all took a good look. It had a picture of Sherlock and John's dear old friend Jim, grinning madly, and underneath it the caption: Moriarty Returns!
I don't remember if Arthur could read or not. But since Merlin could, I assume he could too.