There are various ways Merlin could be injured in fanfiction involving Gwaine and Percival. He might be injured while exploring an ancient and dangerous place. Gwaine and Percival are with him, but they don't anticipate all the traps. Merlin, being the more magically inclined, tries to disarm a magical trap but miscalculates, and it backfires, injuring him. Fanfiction writers like to play with these kinds of unexpected twists to keep the story interesting.
One great 'Merlin injured' fanfiction is 'The Wounded Mage' on FanFiction.net. It shows Merlin getting injured during a battle with a rogue sorcerer. The author does a great job of depicting Arthur's worry for his friend.
Writers often depict Merlin's recovery as a slow and arduous process. They might show him relying on the help of his friends, like Arthur bringing him food and Gaius providing medical care.
Gwaine and Percival could play the role of protectors. They might stand guard over Merlin while he recovers from his injury. They would fight off any threats that come their way, ensuring Merlin has the time and safety to heal.
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.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.
One more idea is that Merlin is tied to the bed in a dream - like state created by an ancient magic. In this dream world, he is faced with his deepest fears and insecurities. His friends in the real world notice his strange behavior and try to wake him up. Merlin, while in the dream, has to find the source of the magic that is binding him and break free. Through his inner strength and the memories of his friends, he manages to do so and wakes up, having learned more about himself and his magic.
If Merlin is a Dark Lord in fanfiction, it could be an interesting exploration of the 'nature vs. nurture' concept. Maybe he turned dark because of the constant fear of being discovered and persecuted for his magic. His actions as a Dark Lord might include taking over Camelot or exacting revenge on those who wronged him in the past. It gives a whole new perspective on his character.
One possible reason could be a magical curse. In many Merlin fanfictions, magic is often used as a plot device. A dark sorcerer might cast a spell on Merlin to make him forget certain things, perhaps to prevent him from using his powerful magic to oppose them. Another reason could be a self - imposed forgetting due to trauma. Merlin has witnessed and endured a great deal of pain and suffering. To protect his own mental state, his mind might subconsciously block out certain memories.