First, you need to combine the two universes' elements well. For example, think about how Merlin's magic would interact with the magic system in Harry Potter. You could also consider character crossovers, like having Merlin attend Hogwarts. Maybe he would be a special student with his unique brand of magic. Another important thing is to plan your plot carefully. It could be a story about Merlin solving a mystery in the wizarding world that even Dumbledore couldn't figure out.
Merlin could be heartbroken. He might retreat into himself and become more withdrawn. After all, he has a deep bond with Arthur, and this sudden break - up would be a huge shock.
Merlin would be very worried. He would probably stay by Arthur's side day and night, using his magic secretly to try and heal him. He might also search for rare herbs or magical items that could cure Arthur.
Merlin is usually very empathetic. He'll listen to the reader's problems without judgment. In some fanfictions, he might share his own secrets and fears to make the reader feel less alone. He'll also do practical things, like bringing the reader food or making sure they have a warm place to stay if they're in a difficult situation. He'll try to show the reader that there are always options and that life is worth living.
One way is to create a power struggle. Maybe Merlin's powerful magic makes some in the wizarding world jealous and they try to undermine him. Or there could be a prophecy that involves both Merlin and a key Harry Potter character, like Harry himself. They have to work together to fulfill the prophecy while facing many obstacles.
Merlin react fanfiction is a type of fan - created story where the characters from Merlin are put in situations where they react to various things. It could be modern - day objects, events from an alternate timeline, or secrets being revealed. For example, Merlin might react to a smartphone in a modern - day AU (alternate universe) fanfic, with his magic causing some unexpected interactions with the technology.
He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
One popular merlin react fanfiction is 'Merlin's Modern Day Reactions' on FanFiction.net. It has Merlin reacting to all kinds of modern technology and social norms in a humorous way.
Arthur might be initially shocked and confused. He's used to Merlin being in control. But then he might find it quite hilarious and start teasing Merlin about it.
She might be completely overjoyed. With tears of happiness in her eyes, she'd immediately say yes and throw her arms around Merlin. After all, she loves him deeply and has probably been waiting for this moment.