By using the portrait as an enigma. If the portrait has a strange expression or a mark that no one can explain, it creates mystery. Also, if the backstory of the portrait is only partially revealed.
A mystery short story can create tension through its characters' emotions. If the detective is frustrated or the victim is terrified, that feeling spreads to the reader. Also, withholding information from the reader until the right moment can build up a lot of tension.
Well, when there are dark settings in a story, it's like a veil is drawn over the normal world. The lack of light hides things. Say, in a dark alley in a story. We don't know who or what is there until something reveals itself. This anticipation builds mystery. The characters are also more on edge in such settings, which adds to the overall sense of mystery.
Use description. For example, describe the locked room as having a strange glow seeping from under the door or making the lock look old and rusty with strange symbols on it. This immediately makes the reader wonder what's inside.
Poe creates mystery by introducing strange events and objects. In his stories, there are often inexplicable happenings. For instance, in 'The Cask of Amontillado', the very act of Fortunato being led into the catacombs for a supposed wine tasting that turns into something far more sinister is mysterious. The use of symbols, like the black cat in 'The Black Cat', also adds to the mystery as the reader tries to figure out what they represent.
In The Dark Tower short story, King creates mystery by introducing elements that seem to defy normal logic. There are creatures and phenomena that are not of the ordinary world. The way time and space work in the story is also a mystery. It's not clear how the characters can move between different places and times so easily, and this lack of clarity adds to the overall sense of mystery in the story.
They often use the vastness and unpredictability of the sea. For example, in 'The Terror' by Dan Simmons, the sea is a harsh and unforgiving environment. The characters are isolated, and there are unknown threats lurking, like the mysterious creature in the ice. This creates a lot of suspense.
One way is through the use of setting. A spooky old house or a fog - covered forest can immediately create an atmosphere of mystery and suspense. Also, withholding information from the reader. For example, not revealing the true identity of a monster or the cause of strange events right away.
Through its setting. A desolate and ruined landscape, like a city filled with crumbling buildings and polluted air, can make the reader feel uneasy. Also, the use of dark and muted colors in the description can add to that sense.
He uses detailed and vivid descriptions of strange settings. For instance, a dilapidated old mansion filled with creaking floors and hidden passages. This immediately makes the reader wonder what secrets the place holds.
First, come up with an intriguing mystery or crime. Then, build interesting characters around it. Also, set the scene and build tension throughout the story.