J.K. Rowling has been hiding a hint to the main theme of the 'Fantastic Beasts' movies for years in plain sight

Jaap Buitendijk/Warner Bros.

Warning: There are some spoilers for "Fantastic Beasts and Where to Find Them" ahead.

"Fantastic Beasts and Where to Find Them," J.K. Rowling's prequel movie series to "Harry Potter," is in theaters now. The film introduces a new Dark magic concept to the wizarding world called an Obscurus.

As revealed in the movie, an Obscurus is a magical parasite that develops over time after a wizard or witch suppresses their magical powers. A person becomes known as an Obscurial when the Obscurus takes. You can read more on what an Obscurial is and how we believe it will play into future "Fantastic Beasts" films here.

Warner Bros.

Here, you can see an Obscurial in the 1926 New York City subway.

While we're introduced to an Obscurial in "Fantastic Beasts," what fans may not realize it that the word Obscurus has been a part of Rowling's wizarding lexicon for years.

Eagle-eyed "Harry Potter" fans noticed Obscurus is a wizarding book publisher which appeared in the work of Rowling's 2001 release "Fantastic Beasts and Where to Find Them."

It turns out Obscurus was a lot more than just a fictional book publisher.

Take a look at one of the opening pages from "Fantastic Beasts and Where to Find Them":

Scholastic

Obscurus Books is listed to work "in association with" Arthur A. Levine Books.

Obscurus Books is listed as being located at 18a Diagon Alley, London. Maybe it will pop up in Universal's fictional recreation of the Wizarding World in Florida or California.

After seeing "Fantastic Beasts," it looks as if Obscurials will be front and center in the five-film franchise. The mysterious Dark magic may also answer a lingering questions fans have from the original "Potter" books.