Merlin keeps a diary, half to vent and half to keep events in line in case they ever become relevant, because so much is happening all the time. He writes it in the language of the old religion, so that no one can read it- obviously, that's pretty damning, but if it was legible then they'd know he has magic anyway, so. Also he lowkey finds it easier to write in then anything else. Anyway, one way or another, pretty early on in the show (like, season 1) Arthur finds the diary, but he doesn't even realize it isn't in (Olde) English; being touched by magic at birth, he can read it. He doesn't even really understand that it's a different language (btw, same when he hears spellcasting- he can tell they're spells because the words seem to echo and hiss, but can absolutely understand the words themselves. He doesn't realize this isn't normal for like....a while.).
He starts reading it mostly to get a laugh at Merlin's expense (Merlin! keeping a diary!), but it opens with a gruesome account of the first thing Merlin ever saw in Camelot being a beheading and how he immediately had some sort of panic attack and threw up, so. Not as fun as he was expecting. He gets conflicted about reading it, properly registering just how much of a violation of Merlin's privacy it is, and puts the diary back where he got it.
But from that point forward...well. He never forgets about it. He never orders Merlin to attend another execution, actually gets in the habit of piling chores on him so he has excuses not to go- or, hopefully, not to notice. Whenever Merlin's being especially shifty and secretive, Arthur is a little more tempted to just go read his bloody diary. He doesn't, of course, but....he's very tempted.
When the magic reveal eventually happens, Merlin....can't talk about it. The mental block is just far too strong for him to say the words he's feared his whole life. He crying tears of frustration just as much as guilt, as he tries to explain it but literally can't. Arthur is freaking out as well, of course, but they manage to communicate just enough for him to understand that Merlin's choked silence isn't willful or defiant- for once. Still very upset about the whole situation, Arthur orders Merlin to stay put and storms up to Merlin's room, retrieving the diary and bringing it back to, presumably, Arthur's bedroom where this is all happening. Arthur goes to sit by the fire- away from Merlin, still huddled by the foot of the bed- and starts reading, not bothering to say a thing to Merlin. Honestly, at a loss for what he could say to Merlin.
Merlin obediently- again, for once- doesn't move from his spot, even though he is obviously itching to know what Arthur's reading. He manages to ask, but Arthur ignores him. He sighs and closes his eyes to try and calm down, since he apparently has time for that-
And wakes up what has to be several hours later, sky lightening outside of the window with the promise of dawn, and Arthur shoving Merlin's diary into his stiff hands. By the time he registers how -bad? But he WANTED to tell Arthur. Disrespectful, he guesses, but cringes at the term- the situation is, Arthur is half asleep in bed murmmering orders of not waking him for at least three hours.
Something something 'you had no right to go through my journal!' 'You had no right to keep lie and secrets from me. from your king." "If you cared that much, you would've read it whenever you apparently found it, then!" "I could've, but I respected our- our friendship enough not to betray you like that. Why would I continue to bother, when you refused to do the same?' (They're assholes, your honor)
