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.
You can try websites like Archive of Our Own (AO3). It has a vast collection of fanfiction, including Merlin fanfiction. Just search for 'Merlin' in the search bar, and you'll find numerous stories to read.
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.
There's also 'A Different Destiny' which takes a different path for Merlin's magic and how it affects his role in Camelot. It's a long - form story that really delves deep into the world of Merlin and the characters we know and love. This story has a large following because of its unique take on the series' concepts.
One common theme is loyalty. The knights' loyalty to Merlin and vice versa is often explored. It shows how they stand by each other through thick and thin.
The characters would likely have a range of reactions. Merlin himself might be both flattered and a bit embarrassed by some of the fanfiction. He's a humble character, so seeing all the attention on him in these fictional stories could make him blush. And for the knights, they might laugh and joke about it, while also being a bit curious about how the fans view their adventures.
He could become a Dark Lord due to betrayal. If someone he trusted deeply, like Arthur perhaps, turned on him because of his magic, it might push him over the edge.