The cat could have a special bond with Arthur too. Maybe it likes to curl up on Arthur's lap when he's in a pensive mood, and this makes Arthur more open - minded about magic. The cat could also be involved in a situation where Arthur has to rely on Merlin's magic, which the cat is somehow connected to, to save his life.
Another possibility is that the cat is actually a transformed being. It could be a person who was cursed and Merlin has to find a way to break the curse. The cat could communicate with Merlin in a special way, like through dreams or by leaving paw prints that form messages.
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.
The physical closeness of Merlin sitting on Arthur's lap can be the start of a deeper emotional connection. Arthur might become more aware of Merlin's vulnerability, which could lead to him being more open and caring towards Merlin in general. For example, he could start sharing his own insecurities with Merlin.
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.
It could start with a slow realization of their feelings. Maybe they were friends for a long time and then suddenly, one day, they look at each other in a new light. After getting married, they become more protective of each other. Arthur might become more open to Merlin's magic, and Merlin would be more involved in Arthur's political decisions.
They could start by having more heart - to - heart conversations. For example, Arthur could share his insecurities about being a king and Merlin could talk about his fears regarding his magic. This would deepen their bond.
It can be shown through his actions too. Maybe Merlin is a bit more snappy or short - tempered with Arthur when he's feeling jealous. Like when Arthur asks Merlin to do a task, Merlin might do it with a bit of an attitude instead of his usual eagerness, because he's jealous of something Arthur has or has done.
It completely changes the dynamic. Instead of just a friendship, it adds a layer of romantic love. Their interactions now have a different undertone. For example, a simple glance between them in the original story might just be friendly, but in the fanfiction, it could be filled with passion and love.
In some versions of this type of fanfiction, Arthur's reaction is complex. He might be stunned into silence when he first finds out. His mind would be racing with thoughts about all the times Merlin has used his magic to save him or Camelot without his knowledge. After the shock subsides, he may have a heart - to - heart with Merlin. They would talk about the implications of the secret, and Arthur would likely start to view Merlin in a whole new light, as not just his servant but as a powerful ally with a great burden.
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.