The Prince Emrys Disguise
BBC Merlin AU where Merlin didn't go to Camelot but a different Kingdom where he also pretty quickly became servant in the royal household and is send with the prince (younger brother of the current king) and a knight and an advisor on a diplomatic mission to keep the peace with Camelot and discuss trading business.
Unfortunately, they have a quarrel with some bandits and the prince dies. In order to not upset the Camelot Kingdom by delaying the visit they make Merlin pretend to be the prince because the Pendragons know the knight and advisor from a previous trip with the prince's older brother before he became king.
Fortunately, Prince Emrys was known to be a smart, witted scholar instead of a brute with a sword. And had a similar body size to Merlin so even the clothes fit almost perfectly.
When they are in Camelot Merlin walks the city by himself in his normal peasant outfit, gets into a row with Arthur, he throws him into the dungeon. An hour later Merlin is set free and brought to the throne room where Arthur has to apologies to Prince Emrys in front of the whole court and delegation.
Apparently, the late prince also was known for wandering his city in peasant clothes.
The advisor is most involved with the treaty about peace and trading and the prince is just there to set his signature on the document.
Prince Emrys is witnessing the feast where Lady Helen sings and almost murders Arthur, but Merlin intervenes there as well as in the tourney in which Vailant is cheating, and when so many people get ill by the Afanc.
The treaty can't be finalized because one of the Prince's knight tries to kill Prince Emrys. The assassination is stopped and upon questioning it turns out that Prince Emrys Brother, the King, wanted him to die in Camelot to have reason for a war. So the late Prince dying on the road was too early.
The assassin knight tells everyone that Merlin isn't the prince but just a peasant and a sorcerer (he figured it out when the assasination attempt failed) but Uther wouldn't believe him. The advisor died in the assassination instead.
So, in order to not admit that he is a sorcerer Merlin has to keep pretending that he is the prince.
Uther sends a message to the other Kingdom that the advisor was killed by the knight and the knight died when he treid to escape. Also, Prince Emrys would stay in Camelot for an unknown time and be an advisor in the Camelot Court. Mostly in how to defeat the magic threats. It's a paid job, as a thank you for saving Arthur and the whole of Camelot before.
No war, but also no peace with that kingdom, also no trading.
Soon after that Lancelot comes to Camelot to become a Knight but couldn't because he's no Noble man. But since Merlin is now assumed royalty he makes him his knight. Of course Lancelot quickly figures that Merlin is a sorcerer but keeps quiet.
Arthur and Merlin need some time to get along but eventually see each other as equals.
Uther is always interested in Merlin's opinion for some reason.
Morgana notes Merlin isn't quite against magic as some others are.
Gwen likes him a lot because Merlin is kind to the other servants and does't mind giving a hand himself.
Gaius had figured out Merlin on day one when he accidentally said that he's from Ealdor.
