There is a forest that has always been avoided by the locals. It is said that within the forest, there are trees that can move and voices that whisper disturbing things. A group of hikers decides to explore the forest. As they enter, they are gradually separated. Each hiker then experiences a series of terrifying illusions related to their past traumas. The forest seems to feed on their fear and pain.
You could start with a character who has a dark past that they are trying to hide. For example, they might have committed a crime long ago. Then, slowly introduce elements that make them suspect that someone is onto them. This creates suspense as the reader wonders if they will be caught.
A dark fantasy story could revolve around a cursed forest. Legend has it that anyone who enters the forest is never seen again. The protagonist, a brave knight, enters the forest to find his lost love. Inside, he discovers that the forest is a living entity filled with twisted trees and strange, malevolent spirits. He has to face his own inner demons as well as the physical threats of the forest to find his beloved.
A simple idea is a hiker gets lost in a dark forest at night. They keep hearing strange howls in the distance. As they walk, they come across a clearing where there is a large, old tree with a noose hanging from it. Suddenly, they see a pair of glowing eyes in the bushes. It could be a werewolf or some other dark forest creature that hunts at night.
One idea could be a story where Dark Yami and Yugi have to team up to face a new, powerful enemy that neither could defeat alone. They start to understand each other's strengths and weaknesses better during their battles.
Another option is a fanfiction about Elvira traveling to a parallel dimension. In this dimension, she discovers a version of herself that is completely different, and she has to find a way to get back to her own world while also dealing with the strange and dangerous things in this new place. It could explore themes of self - discovery and identity.
Another fanfiction could be set in a modern world where Dracula is awakened from his slumber. He is confused and disoriented by all the new technology. A young journalist discovers his existence and decides to help him adapt instead of exposing him. It could be a story about their friendship and Dracula's journey to understand the modern world.
One idea could be a fanfiction where Loki uses his magic to cover up his own dark circles. Maybe he's been up all night plotting against Thor and is trying to look his best in front of the other Asgardians. He could accidentally create a spell that gives everyone in the palace dark circles instead.
Good ideas for a long dark custom story can come from different aspects. For instance, you could base it on a post - apocalyptic scenario where the world has been changed by some unknown disaster. The main character could be a scientist trying to find a cure for a strange illness that is spreading among the survivors. Or perhaps a story about a group of outcasts who find an abandoned town and try to make it their own, facing not only the elements but also other hostile groups. You can also play with the concept of time travel, where a character from the present is sent back to the time when the world started to go dark in the game's lore.