One of the trickier tasks Vin Diesel’s eponymous hero faces in “The Last Witch Hunter” is tracking a villain by his signature scent of “moldering crabapples” — a distinctive enough fragrance in its own right, but hard to separate from the generally funky aroma of decomposition that permeates Breck Eisner’s limp, lame-brained occult thriller. Too drab to succeed even as defiantly unvirtuous trash, this era-straddling tale of an immortal medieval warrior protecting modern-day New York from a Black Death reboot stifles Diesel‘s rough-hewn charisma via a sludgy, impermeable oil spill of CGI effects — in the service of largely unspectacular hocus pocus. Despite a pre-Halloween release date, the pic is more gung-ho than gooseflesh-inclined in genre; either way, it’s unlikely to mint the franchise threatened by its eminently welcome ending.

Commercially, given the extraordinary expanding cultural impact of the “Fast and Furious” series, “The Last Witch Hunter” might expect to ride on Diesel fumes to an extent. Yet if its aim is to reposition him as a solo action star, perhaps re-engaging the dwindling audience for the “Riddick” films, this new vehicle doesn’t really play to his strengths — despite being developed and co-produced by the actor himself. Barring an early (and swiftly discarded) reference to his character’s prowess as a ladies’ man, there’s precious little room here for Diesel’s lunkish, slightly self-parodic streak of humor. And if it’s hard to buy the star as a 14th-century soldier of the Catholic Church, earnestly slaying sorceresses for 700 years without a wrinkle to show for it, the screenplay (bearing evident redrafting scars from a trio of scribes) doesn’t make much of an effort to sell the idea.

For starters, it’s uncertain where our noble witch-hunter, Kaulder, actually comes from: Based on scant evidence in the pre-credit prologue, let’s say it’s the little-remembered European land of Snowsylvania, though eight centuries has been long enough for him to adopt Diesel’s trademark gravelly drawl. It’s probably unwise to demand more detailed a milieu from a film that claims the Black Death plague of 1346-53 was in fact foisted upon humanity by a vindictive Witch Queen (Julie Engelbrecht, or what’s left of her beneath a maggoty digital mask) bent on total human eradication. Luckily, Kaulder — sporting a braided beard and luscious undercut that would not be out of place in contemporary Williamsburg — has our backs, wasting the Queen in a murky introductory battle. Not before she afflicts him with the curse of immortality, however, thus consigning him to a lonely life of winnowing out her mangy kind, haunted by the memory of his long-perished wife and daughter.

He looks good on it, though, even if the on-trend locks are swiftly sacrificed for Diesel’s standard minimalist coiffure. In present-day Manhattan, he lives in relatively comfortable torment in a plush Upper East Side loft, seducing the occasional air stewardess and accepting international witch-hunting assignments from a succession of priestly advisers known as Dolans. The latest of these, Dolan 36th, takes the typically jovial form of Michael Caine, whose helpfully explanatory voiceover is unceremoniously dropped shortly after it begins — and shortly before he’s dispatched to his coffin in mysterious circumstances.

Suspecting foul play, specifically of the “fair is foul and foul is fair” variety, Kaulder begins the search for the Weird Sister responsible, with Dolan’s dormouse-like successor (Elijah Wood, decidedly under-burdened) in tow. From here on, the investigation plays out not unlike a super-sized episode of “Murder, She Wrote,” only with more shape-shifting ghouls and fire-strewn showdowns between good and evil. He also happens upon a spunky good-witch ally in dark-arts club owner Chloe (cannily cast “Games of Thrones” alum Rose Leslie), whose initial surliness and plethora of Hot Topic accessories conceal a rare command of “dream-walking,” a strain of mind-altering white magic. As they follow the breadcrumb trail around a city that seems markedly oblivious to the insect tornadoes and snaking “plague trees” springing up overnight — Pittsburgh fills in unsteadily for the Big Apple, though it’s not the least credible performance here — it becomes clear that the long-vanquished Queen has returned for another crack at earthly domination.

Offering auds nary a wink to the dim absurdity of his mission, Diesel trudges dourly through the proceedings, practically expectorating dialogue that is, in fairness, pretty hard to play with. (“They took all the most powerful witches in the world and put them in one place,” observes one character. “The witch prison,” comes the memorable response.) Leslie brings glimmers of gumption to her conveniently abled sidekick role; she at least appears to be having more fun than formidable thesps like Rena Owen (“Once Were Warriors”), as a stern member of an underground witch-control council, and, most bizarrely, Isaach De Bankole as a blind Ivorian bakery owner and mystic herb merchant named — for reasons of apparent script inflexibility — Max Schlesinger. Perhaps he could have swapped roles with Caine, who appears to be doing some less supernatural dream-walking of his own in several scenes.

There are certainly enough dopey diversions here for “The Last Witch Hunter” to be considerably more fun than it is, but even its most extravagant bouts of silliness are hampered by desultory plotting and Eisner’s oppressively synthetic mise-en-scene. Dean Semler’s lensing has a dull slickness that mostly defers to the pic’s Marmite-colored morass of visual effects, which are somehow as tawdry-looking as they are conceptually elaborate. The pic’s climactic face-off between human and Hexan is a particular letdown, heavily recycling both the character conflicts and the cockroach-and-brimstone imagery of the intro; it’s hard not to wish that Kaulder had simply done a more thorough job 700 years ago and spared us all the trouble. “Can you feel it — your life, your mortality — ebbing away?” bellows the Queen Witch in her final fight. Whether she’s addressing Kaulder or the audience is open to question.