In America at least, detective fiction and horror fiction both sprouted from the same dark soil. Edgar Allan Poe's short story "The Murders in the Rue Morgue" is not just a precursor to every detective novel that was to follow; it was also a typical Poe grotesquerie (spoiler warning: the savage runaway orangutan did it). In the 150-plus years since, the two genres have run on parallel tracks to wind up in a kind of predictable, innocuousness: The horror aisle is now populated primarily by supernatural romance series, and the mystery aisle has become the home for knitting- and cooking-themed series starring brave middle-aged women. Last Days fuses horror and mystery again, returning both genres to their macabre roots.
Last Days revolves around a murder that may or may not have happened in a cult of self-mutilating monks—the idea is that the more you amputate, the closer you get to God. A detective named Kline, who in an earlier case had to cut off his own hand to save his life, is recruited to solve the crime. As he struggles to discover what happened, Kline must keep cutting off his own body parts in order to learn the truth. There is terror there, but it's not just a Saw, Part 13 kind of gross-out: Kline diminishing himself calls back to the long line of detectives who become more battered as they get closer to the truth.
Things continue to fall apart for Kline, and the language and narrative devolve at times into their most basic components: "The door burst open and a man with a gun in place of a hand stood there and there was a rattling and one policeman's head came quickly open to reveal what was inside." By shearing off extraneous elements Evenson removes all but the most necessary and important elements of mystery novels (the quest and sacrifice for truth) and reveals the horror at the core of the hunt.