Another horseman is bringing trouble to Sleepy Hollow during "John Doe."

After a mysterious child arrives in Sleepy Hollow, Ichabod Crane (Tom Mison) and Abbie Mills (Nicole Beharie) have their hands full trying to figure out who he is and what disease has followed him to their town.

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"In this episode, a little kid stumbles into Sleepy Hollow and he is speaking a language no one has ever heard, except for Crane," said executive producer Roberto Orci, according to a new behind the scenes sneak peek. "To make matters more perplexing, his veins are turning black."

Even though the boy is quarantined, the illness starts to spread, eventually infecting Crane as well. No one seems to be able to identify the sickness or how it is transmitted. Meanwhile, the episode's promo shows the young boy running from a horseman, and it isn't Headless. Although this could be Pestilence, Orci points out that the disease may be more supernatural in nature than the characters originally presume.

"What looks like a medical mystery resembles more of a curse," Orci said.

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Recently released stills from "John Doe" show both Crane and Mills walking though a village that looks like it was ripped from the past, a place where all the inhabitants are showing signs of the same, creepy black veins. How the pair manages to get to this place remains a mystery.

Sleepy Hollow airs Mondays at 9 p.m. ET on Fox.