Well, in some 'tortured merlin fanfiction', Merlin is often put in situations where his loyalty is tested to the extreme. There are tales where he is captured by enemies who know of his magic and try to use it for their own evil purposes. This causes him great pain as he has to fight against their control. Also, there are stories where Merlin's relationships, like with Arthur or Gaius, are strained due to misunderstandings that lead to his 'tortured' state.
You can start by looking on fanfiction websites such as Archive of Our Own (AO3). It has a large collection of Merlin fanfiction, and you can use the search function to specifically look for 'tortured merlin' stories. Just type in the relevant keywords, and it should bring up a list of stories that match.
One common theme is the exploration of Merlin's strength in the face of torture. Writers often show how his magic and inner willpower help him endure. Another theme could be the reaction of his friends, like Arthur, when they discover what has happened to him.
You can try searching on popular fanfiction websites such as Archive of Our Own (AO3). Just type in 'Merlin tortured' in the search bar, and it should bring up relevant stories. However, make sure to check the tags and ratings to ensure it meets your expectations.
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.
No. Kidnapping and torture are extremely serious and immoral acts in real life. Fanfiction that involves such themes can be disturbing and may have a negative impact on the readers' mental state. It also goes against basic moral and ethical values.