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 a human girl discovers a hidden portal in an ancient forest and enters a magical realm. There, she meets a handsome elf prince who is initially suspicious of her but gradually falls in love as they go on adventures together to save his kingdom from a dark sorcerer.
There could be a story of a fairy and a mortal man. The fairy is on a mission to find a magical flower that only blooms once every hundred years. The man, a botanist, is also searching for the same flower for scientific reasons. They meet in the enchanted forest where the flower is said to grow. At first, they are rivals, but as they face various magical creatures and obstacles together, they fall in love. Their love is tested when they finally find the flower and have to decide what to do with it, as their reasons for wanting it are so different.
One idea could be a story about two people from different cultural backgrounds who meet while traveling. They initially clash because of their different beliefs and ways of life, but as they are forced to spend more time together due to some unforeseen circumstances like a flight delay or lost luggage, they start to understand and fall in love with each other.
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.
An idea could be a world where magic is illegal, and those with magical abilities are hunted down. The story could follow a young magician on the run. He discovers an ancient, hidden society of magic - users who are planning a revolution. He has to decide whether to join them and fight for the right to use magic freely or try to find a way to live without magic in a world that hates him for it.