As the title suggests, the evil mercenaries are after the Lesser Key of Solomon, which unlocks a demonic prison that will unleash twenty three super-demons into Sleepy Hollow.
"For the Triumph of Evil" rushes into its 'Monster-of-the-week' formula, forcing major character development without the time it would usually take to attain it.
By the end of these episodes, you have your two protagonists, Ichabod Crane, and Abby Mills, realizing that the events from the Book of Revelations have started to happen, and the Headless Horseman is just the beginning.