The Umbral Arcana is the spectral force that prevents New Yorkers from seeing the Unsleeping City. It is created by the Gramercy Occult Society, who uses the Umbral Engine to spin raw Umbra, which naturally occurs in connection to Anti-Populi, into the Umbral Arcana.
Description[]
The Umbral Arcana covers the five boroughs of New York City. When physically seen, it appears as a sort of wispy shadowy fire that surrounds the magical creature or object the Umbral Arcana is hiding. This is seen when Sofia first meets the trolls at the bar, around the door to Kingston's office at St. Owens, and around Santa's Sleigh when the cops arrive to inspect it.
Because of the Umbral Arcana disguising magical occurrences within New York, Santa Claus disposes of his defective clones in the city as to not raise suspicion. This event is known to New Yorkers (Unsleeping City or otherwise) as SantaCon.