It would likely become very strained. Arthur has been brought up to uphold his father's anti - magic stance. So, when Uther discovers Merlin's magic, Arthur might initially feel betrayed and distance himself from Merlin.
The relationship would be in turmoil. Arthur would be conflicted. On one hand, he has a deep bond with Merlin as a friend. On the other, his father's views on magic have been drilled into him. If Uther finds out about Merlin's magic, Arthur may try to defend Merlin at first but would face a lot of internal struggle. He might even be forced to choose between his loyalty to his father and his friendship with Merlin, which could lead to a very rocky period in their relationship. There could be a lot of arguments and misunderstandings between them until Arthur can fully come to terms with Merlin's magic and his own beliefs about it.
If Uther finds out Merlin has magic in fanfiction, Arthur's relationship with Merlin would face a huge test. Arthur is loyal to Uther, but he also cares deeply for Merlin. At first, he might be in disbelief and try to deny it. But when faced with the truth, he would be torn. He could try to protect Merlin from Uther's wrath, but that would go against his father's laws. This would create a rift between them. However, over time, Arthur might start to see the good in Merlin's magic and question his father's extreme views. This could lead to a slow rebuilding of their relationship, but it would be a long and difficult process full of self - discovery for Arthur.
Some fanfictions show Arthur becoming more dependent on Merlin's magic in certain situations. For instance, when facing a powerful enemy that can't be defeated by normal means. This changes the dynamic from just a king - servant relationship to more of a partnership. Merlin, on the other hand, might feel more confident in using his magic openly around Arthur, which also alters their interactions. They could have more equal conversations and decision - making processes.
It would be very complex. Initially, Uther would be in shock. He has been hunting magic - users, and now his son is one. But he might also feel a sense of protectiveness. Merlin, on the other hand, would be confused and angry at being lied to about his identity. Eventually, they might start to understand each other more. Uther could see the good in Merlin's magic and Merlin could see the reasons behind Uther's actions, even if he doesn't agree with them all.
In fanfiction, if Uther finds out Merlin has magic, it could lead to Merlin's immediate arrest. Uther is known for his extreme hatred of magic in the Merlin series. He might see Merlin as a great threat and try to execute him, just as he has done with other magic - users.
It would become much more complex. They might start off being confused and a bit awkward around each other. But then, as they start to explore their feelings, they could become closer in a more intimate way. Their friendship would be transformed into something deeper, with a new level of understanding and trust.
The relationship could become more equal. Before, Merlin was in a somewhat subservient position, but with his magic out in the open, he becomes more of an equal to Arthur. They can now fight side - by - side in a different way, with Merlin using his magic to complement Arthur's skills. This new dynamic can add a whole new layer of depth to their relationship.
Merlin might become even more protective of Arthur. Since Gwen is carrying Arthur's child, Merlin would see it as his duty to safeguard the future of Camelot. He could be more involved in Arthur's personal affairs, which might initially annoy Arthur but eventually he'd come to appreciate Merlin's concern.
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.
One popular plot is that Arthur is initially shocked and angry when he discovers Merlin's magic, but then he gradually realizes how Merlin has been using it to protect him all along. For example, in some fanfictions, Merlin uses his magic to save Arthur from a seemingly impossible battle, and Arthur finally comes to terms with Merlin's powers and they become even closer friends, united in a new way to protect Camelot.
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.