Set sometime after Arthur promises the druids peace, so late series 4. Arthur decides to actually go all out in improving relations with the druids, so he sets out with a small party to meet with them. They talk and work out all that needs to be worked out between them, as Arthur really wants to make amends and forge a lasting peace between them. Then the druids start celebrating, like big bonfire and lots of wine and plenty of very druid things that make Arthur the slightest bit uncomfortable because he doesn't quite know what's going on or what it all means, but Merlin insists that he loosen up and join in. Merlin says these are just the druids' traditions and they can't risk offending them by turning down their offerings, then he scampers off and joins in with obvious delight. Merlin drinks everything they offer him and dances with a bunch of girls and has a grand time, and Arthur sees a more-than-a-little-intoxicated-Merlin get a flower crown from a pretty druid girl kneeling in front of him and then drink wine from the cup she offers him, but he doesn't think anything of it.
The next morning, they're all a little hung over but they did what they came for so they're in high spirits. They're just about to set off for Camelot when they notice a druid girl saddling her horse too like she's going to ride out with them. Merlin goes to ask her why and she says she couldn't very well stay here, not when her husband will be returning to Camelot. Merlin looks baffled and asks who exactly her husband is, and she says him. Apparently the whole flower-crown-and-wine thing was a druidic marriage rite. They girl, Taryn, says she was offered to him as a gesture of good faith and as a way to strengthen ties between the groups. Merlin protests that he's hardly a figure of political importance, he's just a servant, what good does it do to marry him?? Taryn responds in mindspeak that she knows he is Emrys, that there is no one of more importance, and Merlin goes pale. Arthur and the knights are torn between concern, bafflement, and laughing their asses off. Merlin, on the other hand, is kind of freaking out. He is vehemently against the idea of being married to this random girl and denies the marriage's validity. The girl is a bit devastated that her husband doesn't want her, that the great Emrys thinks she's unworthy, and she starts to cry. Merlin struggles between wanting to comfort her and wanting to make this whole situation not be happening. Since Merlin is so panicked and dismayed, Arthur stops laughing and thinks on it for a minute. He takes Merlin aside and explains that what Merlin said last night is still true, that they can't turn down a gift like this without causing serious offense and risking relations that are already a little shaky to begin with. Merlin tries to put up more of a fight, but with Arthur reluctantly opposing him and with Taryn looking desperate to stay with him, he finally gives in, absolutely miserable about it.
Taryn travels with them back toward Camelot. She is incredibly attentive, much to Merlin's discomfort. She insists on making their meal. She serves Merlin and offers to rub his feet. Gwaine is still howling with laughter, of course. Merlin is mortified and keeps turning her away, trying to do so as politely as possible and not make her cry again but apparently unable to convince her that his not wanting to marry her isn't a reflection on her character or her worth as a person. When they reach Camelot, Merlin manages to extricate himself and leave her with Gaius while he takes care of Arthur for the day.
When he gets back, Gaius gives him a look he can't quite decipher but doesn't say anything, which makes Merlin more than a little nervous. Gaius' behavior makes sense when he opens the door to his room to find Taryn naked on his bed. She says it's their wedding night and she wants to lie with him as his wife, and Merlin is like WHOA. He escapes and hides out in Arthur's chambers and Arthur let's him stay in the antechamber for the night. The next day, Merlin sits Taryn down and has a nice long chat about why he can't do this: he did not make the decision with a clear head, he was not properly informed as to what that ritual meant or the commitment he was making with it, she was misinformed about her intended's consent on the issue, he doesn't actually know her and can't justify just marrying her like that, his life is complicated enough without this. Taryn takes his hand and says that she just wants to help him. She doesn't have to be his wife if he truly doesn't want her to be, but she can at least be his friend. She can help him bear the burdens of his secret, since he has so few to talk to and even fewer who truly understand what being Emrys means and the pressure it puts on him.
Idk if this would end up with Merlin and Taryn actually falling in love or if Merlin would find a tactful way to dissolve the marriage because it wasn't consummated, if Taryn would go back to the druids or stay on in Camelot.