In a fanfiction, Merlin's pain can be feigned through his magic use. Maybe his magic falters when he's in pain. For instance, a simple spell that he usually does with ease might be more difficult or not work as well. You could also describe his body language, like hunching over as if to protect a part of his body that hurts. His reactions to sudden noises or movements can also be a sign of his pain, like flinching.
Feigning Merlin's pain can drive the plot forward in many ways. It could lead to a search for a cure if others notice. This search can take the characters on adventures, introduce new magical elements or characters. His pain can also be a factor in battles or important events. If he's in pain during a battle, it can change the outcome and the way the other characters fight. It can also lead to emotional moments between Merlin and those who care about him, deepening their bonds.
One interesting plot could be Merlin hiding his pain from Arthur because he doesn't want to seem weak. He might endure the taunts of the knights in silence while secretly using his magic to heal his internal wounds. Another could be Merlin hiding his pain over his mother's absence or the fact that he has such a huge responsibility on his shoulders with his destiny to protect Arthur and Camelot.
The main character is Merlin, of course. He is the focus as he is the one with the hidden pain. Arthur Pendragon is a key figure too. Since Merlin is Arthur's manservant, Arthur's actions, words, or even his ignorance of Merlin's true self can be sources of Merlin's pain. Sometimes, the Great Dragon can also be involved. The dragon might be aware of Merlin's pain and offer some cryptic advice that Merlin has to decipher while still hiding his pain from others.
Merlin's relationship with Arthur could become even stronger. They might have more adventures together and truly become equals in their fight for Camelot. Their bond could be tested, but in the end, it would only grow deeper.
Well, one prediction could be that Merlin discovers a new form of magic. Maybe he'll find an ancient spell book that has been lost for centuries. This could lead him on a journey to master this new magic, which might be different from the magic he already knows. He could face challenges along the way, like dealing with other wizards who are jealous of his discovery.
In a more humorous take on the 'Merlin tied to a bed' fanfiction, it could be that Merlin has fallen ill and the overly - protective Gaius has tied him to the bed to make sure he rests. Merlin, being the active person he is, complains constantly. Arthur comes to visit and teases Merlin about finally being still for once. Meanwhile, Merlin tries to convince Arthur to let him loose, promising that he'll be fine. Eventually, Gaius relents and unties Merlin, who then immediately starts bustling around the room.
He would become more brooding. As a vampire, he'd have to deal with new emotions like the constant thirst for blood. This would make him more withdrawn from his friends in Camelot at first.
In Merlin elf fanfiction, Merlin's character can develop in many ways. He might become more in tune with his magical side as he interacts with the elves. Since elves are often depicted as having a deep connection to magic, Merlin could learn from them to control his powers better.