CBS

Season 4: Episode 18 - Omnivore

Omnivore: Hotch is summoned to South Boston to the deathbed of a detective who headed the first case Hotch worked in the BAU. The dying cop confesses that he made a secret deal with the serial killer. If he stopped looking for the UnSub, the UnSub would stop killing. For ten years both parties observed their unique contract. But now, when the death of the detective triggers the return of the UnSub, Hotch and the BAU must travel to  Boston to apprehend the most notorious serial killer in New England's history.