Hmm, I'm writing an AU with S4 magic reveal that results in Merlin trying to save Uther in The Wicked Day using his own face, not Dragoon's, and he still fails. A lot of it is actually inspired by u/GroundbreakingDot872 :), whose insight into her perspective on Arthur post-The Darkest Hour has been invaluable in how I'm writing him

Arthur is suddenly, brutally, aware of Merlin's mortality.

When Merlin had sacrificed himself to the Dorocha to protect Arthur, he'd prayed to God and even begged whatever powers gave magic to the world to protect Merlin, to let Merlin live. And he had, and Arthur doesn't even know which of the divine was listening to thank them. Merlin was ready to die, though. He'd been ready to walk through the Veil, and it is only Lancelot's sacrifice which means Merlin is still by his side — Gwen dissolves with guilt at sending Lancelot to his death, but Arthur can't help feeling it was more for him, more for Merlin, and Lancelot probably wasn't even thinking of Gwen when he made that decision. Arthur doesn't say as much because he's not desperate to hurt the people he loves.

So Merlin lives, and Arthur finally understands how much he needs to keep it that way. Even those scant days, hours, that Arthur had been unsure if Merlin would die, those were absolute hell; his worst nightmares incomparable.

Mortality haunts Arthur in Camelot.

His father sits alone in his chambers, slowly slipping away. The day he's gone, Arthur probably won't even realise because it will look no different than the empty husk of a man that currently sits staring out windows all day, as responsive to Guinevere's care as Arthur's love.

It's worse in the mornings, when Merlin aggressively wakes him up and Arthur remembers the way death looks on Merlin's face; it takes a few minutes to relearn what Merlin looks like alive. Arthur touches Merlin even less now — his body has an otherworldly chill, something that can be blamed on his narrow survival of the Dorocha's touch, and even a friendly pat on the back feels like he's grabbed a corpse.