A single popular subgenre can get a bit formulaic over time. One of the best ways to spice things up is often to combine a couple of beloved storytelling concepts. Throw ghosts, vampires, and demons into an existing structure, and everything changes. The Occult Detective is the default hero of the beautiful marriage between detective stories and supernatural fiction.
Since the early days of Sherlock Holmes, episodic stories about detectives solving one case at a time have been easy hits. However, not everyone is interested in the affairs of angry people murdering each other. Some would rather see a detective investigate the unseen world of fantasy noir.
Among character archetypes, the Occult Detective may be the easiest to draw from memory. They appear disheveled, they're well-dressed but poorly kempt, they smoke like a chimney, they talk tough, and they're always clad in a full-length trench coat. An Occult Detective typically exists in an Urban Fantasy setting. If the fantasy elements are under wraps, the detective is a key part of hiding the secret and keeping both worlds safe. If they're out and about, then they function like a typical fictional investigator. They typically know everything there is to know about magic, either practicing it themselves or using their advanced knowledge to unravel the power of others. The Occult Detective is rarely an unmixed hero, there's almost always a bit of moral ambiguity to their actions. They have a great deal of overlap with monster hunters, but they tend to take a more intellectual approach to similar problems. Real-world Occult Detectives are often silly, but the ones on the page and the screen inspire awe.
Bram Stoker is often credited with the creation of the first Occult Detective character. His Dracula introduced Abraham Van Helsing, a doctor who swiftly morphed through pop-cultural iteration into a monster hunter. However, Stoker was actually inspired to create this by the same author who wrote the real first vampire story. Irish writer Joseph Sheridan Le Fanu wrote several excellent short stories and novellas, many of which didn't get the attention they deserved upon their initial release. Le Fanu re-released a few of his classics in an anthology piece called In a Glass Darkly in 1872, the year before his death. The original lesbian vampire novella Carmilla was included in the collection. The short story collection framed the tales as papers from the collection of the late Dr. Martin Hesselius, the first full embodiment of the Occult Detective trope. Stoker's Dracula popularized both vampires and the Occult Detective, but Le Fanu laid the groundwork.
The Occult Detective found several homes in the world of comic books. DC published many of the best-known examples, but they also lay claim to one of the first. Doctor Occult was created in 1935 by Jerry Siegel and Joe Shuster, who went on to create Superman only three years later. Doctor Occult is the oldest character in DC's backlog to still appear in comics. Fifty years later, DC would introduce John Constantine, the ultimate distillation of the Occult Detective trope into a single character. Constantine is the central DNA strand of just about every example of the concept since he came out. He was created by Alan Moore as a side character in Swamp Thing, but his Hellblazer solo run made him iconic. Everyone from Harry Dresden to Hellboy has taken a bit from the volumes of John Constantine.
There are countless TV shows about Occult Detectives. Constantine had a short-lived series of his own in 2014, but he went on to appear throughout the CW Arrowverse. The same network hosted the long-running and beloved series Supernatural, which saw its protagonists blur the line between hunter and detective. The X-Files often sets Mulder and Scully up against fantasy elements alongside its steady diet of aliens and government conspiracies. Though less popular, the 2011 series Grimm attempted to bring the traditional police procedural gimmick into the realm of the supernatural. Even anime series like YuYu Hakusho bring up the idea, though the main character mostly just beats up whatever supernatural concept he encounters.
The Occult Detective is an archetype so recognizable, that it could fit into just about any supernatural setting. They all seem to have a lot in common, but it's been a gradual process of developing the ideal example. If Le Fanu had drawn Dr. Martin Hesselius in his time, one wonders whether it would've looked anything like John Constantine. If the character appeared in a film today, he'd almost undoubtedly don a trench coat and smoke non-stop. With both detective fiction and supernatural fiction riding high in the age of streaming TV, there's no sign of stopping for the Occult Detective. Someone needs to get out there and solve the supernatural, and only a certain type of character is right for the job.