The Hollow Ones: Hogan, del

A horrific crime that defies ordinary explanation.
A rookie FBI agent in dangerous, uncharted territory.
An extraordinary hero for the ages.
FBI agent Odessa's life is derailed when she's forced to turn her gun on her partner, who becomes suddenly, inexplicably violent while apprehending a murderer. Devastated, Odessa is placed on desk leave pending a full investigation.
But what troubles her most isn't the tragedy itself - it's the shadowy presence she saw fleeing his body after his death.
Questioning her future with the FBI, and her sanity, Odessa accepts a low-level assignment to clear out the belongings of a retired agent. What she finds there puts her on the trail of a mysterious figure who claims to have been alive for centuries, and who is either an unhinged lunatic... or humanity's best and only defence against unspeakable evil.