One way is to create a new character who is sick. This character could be a key figure in the story, like a healer who is unable to use their powers because they're ill. It adds a layer of drama and conflict.
Here are some basic steps to add a sick (by which I assume you mean intense or dark-themed) Elsword fanfic. First, decide on the main plot elements that will make it 'sick', like a really tough and dangerous quest that the characters face. Then, focus on the characters' emotions during this difficult time. For example, if Elsword is in a fight against a powerful and evil enemy, describe his fear, determination, and how he might be pushed to his limits. Also, add some unique settings that enhance the dark and intense atmosphere, such as a spooky, abandoned castle filled with traps and secrets.
How about a 'Time - Sickness'? This could be a condition where the affected character experiences time differently. They might age rapidly or be stuck in a time loop, which would be really challenging for them and their friends to deal with in the context of the Elsword world.
Ain would also be interesting. His mysterious past and his unique abilities could add a new layer of complexity to the story. He might be an ally or an unexpected foe, and his interactions with Elsword and Elesis could lead to some intense plot twists. Maybe he has some hidden knowledge about their destinies that he reveals gradually.
Elsword Light Novel is mainly about the adventures in the Elsword world. It follows the characters like Elsword and his companions as they battle against various foes, explore different regions, and face challenges that test their strength and friendship.
First, really know the Elsword universe. Study the characters, their abilities, and the world settings. Second, make the reader insert character relatable. Don't make them too overpowered right away. Third, focus on the plot. It could be an adventure within the Elsword world, or a relationship - building story with the existing characters. For example, if you're writing about the character Elsword himself, think about how your reader - insert character would interact with his brave and determined personality.
I think the key is to have a solid understanding of the story and characters. Make sure the plot and art are coherent and don't introduce sudden, confusing elements.