Cross-genre horror, also frequently referred to as slipstream, are tales which bend genre definitions, but still contain some elements of horror. Authors of cross-genre fiction will combine elements from two or three different genres to create a cross-genre. A great example of this is the Star Wars movies, which are a blend of science fiction and fantasy. Cross-genre horror will always contain elements of horror. Recently, paranormal romance, a cross-genre blend of horror and romance, has been very popular. Other cross-genre blends include science fiction horror, which would include subgenres such as rampant technology, man made horror, and zombies, horror mystery, which combine elements of mystery and horror, such as crime horror and serial killer horror. Mary Shelley's Frankenstein is a good example of science fiction cross genre horror, and a great blend of horror, mystery, and science fiction is Robert Louis Stevenson's The Strange Tale of Dr. Jekyll and Mr. Hyde. Horror comedy is another popular cross-genre that has its own subgenre.
Cross-genre horror is not a new concept. In fact, horror has almost always been blended with other genres. Jane Austen's Northanger Abbey or Charlotte Bronte's Jane Eyre are romances with elements of gothic horror in them. When weird tales were popular, they were often crossed with science fiction and mystery, sometimes, weird tales were even crossed with the western genre. The result was weird westerns, such as Joe Lansdale's Dead in the West.
That being said, the balance of the blend is very important to sustaining a cross-genre horror. Too much of the horror element may counteract the other elements. For example, a paranormal romance will quickly cross the line into just plain horror if too much blood and guts are described. Also, science fiction horror can easily become just science fiction if the technical aspects of the work are explored more fully than the horrific elements. Cross-genre horror is when the scares come at a perfect blend with the other elements of the story.
Level of Characterization: The level of characterization in cross-genre horror varies as the subgenre is so vast.
Level of Plot: Expect moderate to high level plots from cross genre horror as several different elements are woven together to create a compelling story.
Level of Supernatural: The level of supernatural in cross-genre horror varies depending on which genres that horror is crossed with, but generally expect supernatural occurrences to be moderate to high.
Level of Scary: Scary is another variable level in cross-genre horror and depends almost entirely on which genre is crossed with horror. For example, as a rule, paranormal romance is not going to be incredibly scary.
Level of Violence: The level of violence in cross-genre horror is also variable depending on the type of genre that horror is crossed with.
Typical Setting: There are no typical settings for cross-genre horror as a whole, though, small towns and generally isolated settings seem to feature prominently in this subgenre.
Because cross-genre horror is a blanket term for many subgenres, such as horror comedy, paranormal romance, dark mystery, and weird westerns, several subgenres can be related to cross-genre horror. Also weird tales and gothic horror, which could be considered precursors to cross-genre horror would be related. Man-made horror, zombie, vampire, and alien horror all have elements that can be related to cross-genre horror as well.
Blends. Because of the blended nature of the subgenre, don't read in this subgenre if you are looking for straight-up scares. Extreme horror is rarely found in cross-genre horror because of the balanced nature of the different elements.
I Am Legend by Richard Matheson- Part vampire and part zombie, the monsters in this novel were the results of a pandemic spread through biological warfare. Robert Neville, the seemingly sole survivor, documents his efforts to cure the disease in a post-apocalyptic wasteland. This story was one of the forerunners of the zombie genre.
Ancestor by Scott Sigler- When a team of scientists is tasked with recreating the first mammal, they are unprepared for the results which could wipe out all of humanity.
Necroscope by Brian Lumley-This series opener is about a psychic spy who works to rid the world of vampires.
Dead in the West by Joe Lansdale- Reverend Jedediah Mercer finds himself embroiled in a conflict with zombies and the medicine man who leads them when he rides into the town of Mud Creek, Texas.
The Strange Case of Dr. Jekyll and Mr. Hyde by Robert Louis Stevenson- This novella is part mystery, part horror, and even a little bit science fiction as lawyer, John Utterson, struggles to find the connection between his client and friend and the violent Mr. Hyde.
Tommyknockers by Stephen King- When a writer stumbles upon the remains of an alien spaceship in the woods outside a small town in Maine, the relic has bizarre and violent effects on the townspeople.
Who Goes There by John W. Campbell- In this science fiction novella that was later adapted into the horror film, The Thing, a crew of scientists discover a shape-shifting alien in Antarctica and must fight to protect humanity, but paranoia and suspicion work to destroy the group as anyone could be the alien.
Book of Tongues by Gemma Files- The first in the Hexslinger series combines western, horror, and romance. Edward Morrow goes undercover to investigate hexslinger, Asher Rook, but his only hope for survival is Rook's lieutenant and lover, Chess.
Dead Until Dark by Charlaine Harris- Part mystery, part romance, part horror, this first novel in the Sookie Stackhouse series is the inspiration for the popular show Tru Blood. In this novel, vampires are very real, and a telepathic waitress joins forces with a vampire in order to track a serial killer in her small town.
Odd Thomas by Dean Koontz- A short order cook who can see ghosts is suspicious when he finds a large number of shadowy creatures surrounding a new stranger in town. As Odd investigates, he realizes that he and his odd band of ghostly allies may be the only ones able to stop an impending catastrophe.