Myth, the Universe, and Everything

I may be off-base, but I think folks who dig on the darker stuff (terror, protagonists under intense stress, high body count) would get into the Pendergast series by Douglas Preston & Lincoln Child; they're emphatically not fantasies, except for maybe the last one, but the fact that an immortality potion or a drug that turns you into a rampaging monster is explained with some pseudoplausible scientific jargonwaving doesn't mean your books aren't fantastical in their effect. :)