Merlin knew Arthur would return. One day. He was, after all, The Once and Future King. When Gwen, the last of his friends died, he stayed strong. He refused to turn to drink, or to darkness. For hundreds of years, Merlin waited. For the first lifetime, he attempted to make friends, but watching them die was far more painful than not having them to begin with, so by age 112, he had resolved to hide alone, deep in the forest. Unfortunately, as modern-day arrived, hiding in the woods became impossible. Everyone needed documentation, a listed place of residence. He could no longer hunt without a permit. really he was not allowed to exist independently at all. So he created a fake birth certificate, which in turn meant he could no longer drink potions to seem younger than he was, at least not regularly.
The Bubonic Plague didn't bring Arthur back, nor during The Inquisitions, nor either of the World Wars.
Even after nearly 900 years, Arthur seemingly died a few days ago. Gaius turned to eternal sleep with a smile. Gwen the same, in Merlin's arms, with all of the young, unrecognizable knights surrounding them. Merlin's mind did not slip. He needed to remember every moment of pain. So that when Arthur came back, he'd be ready. What he did not expect, was now when the earth was supposedly in a time of peace, to walk straight into him in the middle of London. He was still wearing his armor, but it was clean. Merlin thought this was a dream at first, or a nightmare. That he'd wake up and he'd be gone, or that he'd try to reach for Arthur but his hands would go right through him. For fear of this, he resisted touching him. Arthur stared blankly at him for a moment, before a click of recognition came into his eyes.
"Merlin?" Merlin didn't move.
"It is you, I saw you in this form just a few days ago." Arthur looked around as if seeing his surroundings for the first time. He grabbed merlin by the neck and brought him into a huddle so no one could hear them.
"Where the hell are we?" Merlin continued to just gape at him. Finally, after Arthur had just about strangled him, he said.
"This is London," Merlin said He was beginning to wonder if this was real. Arthur's eyes were huge with confusion. Staring at the trucks, at people's clothes, their phones.
"I think I'm gonna be sick," Arthur responded after a few moments.
"Yeah? Me too." Merlin brought Arthur to his tiny flat. Very little of the money he had was earned. When this identity was younger, he worked at a hospital, but they frowned upon having such an old physician, so he had to retire. Now, most of the money he earned to stay near Arthur's place of rest, was either forged with magic or stolen with magic but even with this, his home was small. Arthur passed out just as he lay him on the couch. Merlin imagined rising from the dead had to be exhausting, and living after being dead for 900 years had to be disorienting. So he allowed Arthur to sleep. During this time, the realization hit him. He sat on the floor of his room and did an odd mixture between crying, screaming, and laughing.