The Reaper Follows by Heather Graham

Death comes for everyone.

Deep in the Florida Everglades, the body of a woman is discovered in pieces, presumably ravaged by an alligator. Upon closer inspection, it’s determined no animal could make such perfectly precise cuts. Only a blade could do that. Wielded by a human. Soon, dozens of oil drums emerge amid the river of grass. Each one packed to the brim with body parts.
FDLE special agent Amy Larson and her partner, FBI special agent Hunter Forrest, share a bad feeling that extends beyond the horrifying nature of the grim discovery. They’ve seen this kind of sadistic killing before, and when a small beige horse is discovered at the bottom of one of the barrels, they know exactly what it means. The fourth horseman of the apocalypse rides a pale horse—and his name is Death.
With so many bodies to identify, connecting one victim to the next is easier said than done. But finding a pattern in the chaos might be the only way Amy and Hunter can zero in on the killer, testing their skills as agents—and their relationship—like never before. And when the disturbing trail of clues signals these slayings are just the beginning, the agents will have to return to where it all started before it’s too late. The apocalypse is coming, and Hunter and Amy have only one chance to stop it, even if it means sacrificing each other.