It is 1812, Napoleon is advancing imperturbably toward Moscow in his Russian campaign, and four Russian officers have made a deal with Wallachian mercenaries with a brutal reputation to disrupt his supplies. Only one of the officers knows the true source of the mercenaries' abilities - although even the most marginally intelligent reader will have already picked up on the mention of Wallachia (and the master of the mercenaries' choice of name - Zmyeevich (translated for us by the narrator as "son of the serpent, or dragon")) and added all the other clues together to make V. V for Vampire, or (in Russian) Voordalak.
But our knowledge of what these brutal killers - nicknamed the Oprichniki by their employers, after the more human, but equally brutal historical bodyguards of Ivan the Terrible - really are isn't the point. What we are really exploring is the state of the narrator, Aleksei Ivanovich Danilov, in his connections with his three comrades, with his lover (and his physically distant wife), and with Russia and humanity itself.
Afterall, the main difference between the undead and the living (for the most part) is that the undead are entirely lacking in empathy, even with one another. They are not even animal - for pack and herd animals care for their group members - in some sense, they are the antithesis of this.
And so, Aleksei learns about what it is to have trust, and faith, in his fellow humans - to accept that is the most important thing about being human, in fact - and incidentally kills off 12 brutal predators out of myth, although not without the usual and expected assaults on both his spiritual and physical state (and a good couple of twists, most of which it is possible to spot before the narrator does, I believe intentionally).
Wisely, Kent keeps Dracula out of it, only allowing him onto the stage as the pseudonymous master of the Oprichniki before he departs homeward (only to reappear safely in a single flashback later in the book). I almost wish that he'd not even allowed that much of the Count to appear - so much of the mythos of the modern Vampire story is built around the adaptations and extensions of Stoker's novel (and the Hammer Horror films) that even a brief appearance risks overshadowing the rest of the characters. Luckily, his appearance is, indeed, brief, and his apparent protege (himself delighting in the pseudonym of Iuda - Slavic "Judas") manages to amply animate the final third of the story at least.
It also helps, of course, that the Russian campaign itself presents a solid historical backdrop - the occupation of Moscow, especially, also admitting of various metaphorical comparisons with the state of undeath - pulling "Twelve" safely away from being Just Another Vampire Story.
All in all, "Twelve" is more than a cut above the majority of "supernatural" fiction, and definitely deserves a look.