Merlin might have used his knowledge of the castle's secret passages. He could lead Arthur through those passages, away from the assassins. Also, Merlin could have called upon some magical creatures he's befriended, like the dragon, to help him. The dragon could fly in and either scare the assassins or carry Arthur to safety.
Another way Merlin could save Arthur is by disguising himself as Arthur. Since he has magic, he could easily change his appearance. He'd then draw the assassins away from the real Arthur. Merlin would be putting himself in great danger, but his loyalty to Arthur would make him willing to take that risk. He could then use his magic to fend off the assassins once they realize they've been tricked.
One great Arthur Merlin Assassin fanfiction is 'The Hidden Blade'. It weaves a thrilling plot where Merlin uses his magic subtly in the role of an assassin to protect Arthur. The story has great character development and keeps you on the edge of your seat with its action scenes.
The popularity might be due to the unique combination of characters. Arthur and Merlin are well - loved characters, and seeing Merlin as an assassin gives a fresh twist to their relationship and the overall story. It adds an element of danger and mystery.
The banter between Merlin and Arthur in fanfiction is really interesting. It can range from light - hearted jabs at each other's habits. Merlin might poke fun at Arthur's love for jousting and how he always gets a bit too competitive. Arthur could retort by mentioning Merlin's clumsy attempts at serving him. Their banter also reflects the underlying respect they have for each other despite their different social standings. It's like a friendly sparring that keeps their relationship dynamic.
The 'Redemption' trope is also seen in some fanfictions. Maybe Arthur has made some mistakes in the past, and Merlin helps him to redeem himself. Or vice versa. It's a way to explore the characters' growth and development. And then there's the 'Battle Against Evil' trope where they fight side by side against dark forces, which really showcases their bond and loyalty.
I really like 'The Sword in the Stone' as a fanfiction - like take on Merlin and Arthur. It shows Merlin as this wise and mysterious figure guiding Arthur on his journey to become king. There are also some great interactions between the two characters that show the deep bond they share. You can find it in many libraries or online bookstores.
One consequence could be that Arthur becomes more aware of the threats around him. He might start to trust Merlin even more, seeing him as someone who truly has his best interests at heart. This could lead to Arthur relying on Merlin more in future situations.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
During the time of Merlin's captivity, they could have long conversations that they never had before. Arthur might open up about his insecurities regarding magic and his kingdom, and Merlin could share his true feelings about serving Arthur. This would strengthen their bond in a new and unexpected way.