At first, Merlin returns to Camelot because it’s only fair to Gaius and to Guinevere. But it’s a difficult burden to bear, and the survivor’s guilt eats at him in a way it cannot possibly hurt others who loved Arthur. He failed as no one else had, failed to fulfill his destiny and failed to protect and serve as was his will and duty. Still, he grins and shoulders the weight because Guinevere needs his help to reinstate magic into the land. As time passes, Guinevere begins to heal from the loss and Merlin notices that Leon really doesn’t leave her side. He imagines that Gwen is so much stronger than he ever realized, because for him Arthur’s loss is still a fresh sting. Perhaps a phantom limb of a life that was meant to be more.
When Nemeth finds itself in trouble and Mithian begs his help, he goes to her with haste, failing to convince himself that he’s riding out to help a friend and not because he’s fleeing Camelot. He goes to Mithian and fights to protect her and Nemeth like he couldn't protect Arthur and Camelot and they become fast friends because she gets it. So he stays. Because the thing is- the thing is, he misses Arthur every single day but if he doesn’t keep Albion safe until Arthur returns, then no one will. And one day, Mithian turns to him at dinner and asks if he will stay longer, and he says yes - until such time she no longer has need for him. He doesn't want to go back to Camelot anyway, Gwen doesn’t need him anymore and Gaius has passed; so Nemeth is as good a home as any. When Mithian asks him to marry her he says yes. Even though she isn’t his other half, he loves Mitzi’s determination and inherent goodness anyway, and the fact that she understands somehow that his bond with Arthur was so much more than it looked at surface. He loves her, he thinks. When she has their son and he asks her what she’ll name him, she looks at him in that way she has and says like it’s the most obvious thing in the world, “Arthur, of course.”
Little Wart is shaping to become almost a good a warrior as his namesake, but he dies young from a terrible sickness that takes a great many all across Albion. Merlin despairs that this tragedy wasn’t enough to return the Once and Future King, and he wonders what will qualify. He despairs that he hadn’t learned enough from Gaius or about magic to stop the sickness. He travels, far, far, far from Nemeth and from Camelot, seeking and learning everything he can. He collects a young ex-ruffian apprentice on the way, to whom he tries to teach what he knows (and from whom he learns a lot without meaning to). The apprentice is from one of the western kingdoms and sometimes calls Merlin Master Hawk, because his people didn't have a word for falcon. Eventually Merlin tells him the story of Arthur, even little secrets (like the fact that Morgana used to call him Wart when they were both children and that Arthur had two dogs that he loved dearly and that he was surprisingly artistically gifted). When Merlin finally returns to Nemeth, Mithian looks at him and wonders that he always looks the same, and her fingers go to the shock of gray in her hair before reaching out to touch his ever inky dark hair. His apprentice simply shrugs next to him and says “well you’re Emrys, aren’t you? In my kingdom’s tongue Emrys means immortal. Didn’t you know?”
Merlin ends up staying at Gwen’s bedside first when she passes, followed by Percival, who had followed Merlin to Nemeth and become a knight there, Gwaine’s memory too strong in Camelot. Mithian dies next, and the kingdom feels her loss keenly. She has bequeathed her kingdom on a trusted cousin, knowing that Merlin would not want the responsibility of a kingdom to bear on his own. He keeps memories of these friends tucked in a place in his heart near where Arthur lives, and he figures out how to magically revisit these memories as fresh as the day they were made. He sometimes watches Gwen smiling and introducing herself to him and Gwaine helping them in the tavern brawl and Mithian speaking to him on her first visit to Camelot. And if he locks away his Pensieve at night only after watching Arthur jeering at him at their first meeting, or asking his help from across a fire, or seeking his eyes out in the throne room after the coronation, well, then - nobody was alive to say anything anyway.
He leaves Albion altogether, youth in his step but not in his heart; he travels and meets new people and hears new stories and learns one day that his old apprentice had started a school for magical children. O ne of the students tells him that it was named Hawk’s Wart and Merlin feels a rush of affection for little Godric. He enchants the castle named after Arthur with every protection he can imagine and smiles when the children take to calling it Hogwarts after a time. He still keeps his nomadic lifestyle and mitigates small disasters where he can but he keeps an eye on the school on the hill just as he keeps an eye on the tower on the lake beside it.