Headcanon that Merlin keeps notebooks and writes down everything - everything - he wants to show and tell Arthur whenever he comes back
It starts off as a project. He writes down important moments, things he knows he will have to inform Arthur of if when he comes back, but one day, Gwen tells him a joke and he can’t help thinking it would have made Arthur laugh. It was a bittersweet realisation, so he wrote it down to share with Arthur whenever he saw him
Slowly, more jokes were added, and anecdotes, books he wanted to tell him about, plays and music he was sure Arthur would enjoy, and eventually films and shows
Sometimes, he writes when he particularly misses him
He keeps these notebooks on a large secondhand bookshelf, and tries to keep them in chronological order, although the earlier ones aren’t in the best condition after a thousand or so years, but he’s enchanted them to stay legible
It becomes a habit. He decides, subconsciously and superstitiously, that once he has filled the bookshelf, Arthur will return. Merlin does, and Arthur doesn’t. He buys a second bookshelf, and then a third
Finally, Arthur is back and Merlin excitedly showing him his collection and telling him there’s so much he has to share. Arthur is amazed, “you wrote all of this for me?”
Merlin realises suddenly how overwhelming it must seem. “It made it easier, not being able to share all these moments with you. It felt like I was sharing them with you in a way. It was like I was writing you a letter.” He shakes his head. “But you’re here now, it’s silly! You don’t have to read them at all. I can go over them and pick out the important parts, the things you’ll need to know and—”
Arthur stops him. They go through each book together. Merlin hadn’t realised how many good moments he had forgotten over time, and he gets to relive them with Arthur at his side, both chuckling along as Arthur asks questions, “you did what?”
They aren’t all nice stories. Merlin winces as they get to the wars. Arthur holds his hand as he recounts each loss of a friend, and Arthur hates to see Merlin grieve, but he’s glad that he made friends throughout his life because he couldn’t bear the thought of Merlin being entirely alone while he waited for him
But even if he wasn’t always alone, he was lonely. The notebooks proved as much, and sometimes Merlin still panicked if he woke up and Arthur wasn’t close. Eventually, Arthur slid into Merlin’s bed beside him, only to keep him calm. It was the best Merlin had slept since- well, he couldn’t remember
Sometimes one of them would wake up with nightmares of the wars and battles and deaths and wounds, and the other would hold them until they fell back to sleep. Merlin kissed Arthur’s forehead one night without meaning to and froze, but Arthur had only burrowed deeper and fallen asleep with a soft smile
“So that was the last book,” said Arthur as Merlin finished the final page
“Yes, I wrote that the night before you came back,” said Merlin
“I’m all caught up,” said Arthur. “There’s nothing more to say.”
“No, no more to say,” Merlin agreed absentmindedly
A silence fell over them. Who moved first, it was impossible to tell, as if they had both decided in the exact same moment that it had to be now, and they kissed, pulling each other close
“I love you,” said Merlin once they eventually parted
“I got that,” said Arthur, gesturing to the pile of notebooks surrounding them. “The longest love letter in history,” Merlin laughed before Arthur added, “I love you, too”
Edit to add: the last show Merlin wrote that they watched together was Heartstopper, thanks
