The publisher made the announcement with a 14-second teaser trailer, which you can watch above. In it, a character seems to be repeating the phrase "must sleep."
The Assignment is a story-driven mission centered on Juli Kidman, a side character from the main game. The downloadable content is part of The Evil Within's $19.99 DLC season pass, which also includes two other add-ons.
For more on Tango Gameworks's The Evil Within, which was released last October on PlayStation 3, PlayStation 4, Windows PC, Xbox 360 and Xbox One, check out our full review.
Update: A Bethesda representative reached out to Polygon to say that the company has switched up the order of the add-ons from what was listed in the season pass announcement. The Assignment will focus on Juli Kidman, not the Keeper/Boxman. We've edited the story to reflect this.