This encounter is designed to be a challenging/deadly encounter for low-level players—a kind of mini-boss fight. It is purposefully designed to be difficult and something new for more experienced players.
The settlement the PC's find themselves in is located in a 'soft place,' a location on the Material Plane where the border between the other planes is incredibly thin, and occasionally non-existent. This particular soft place connects the Material Plane and the Elemental Plane of Fire. An eldritch horror called a Morhell has accidentally passed through. Confused by its surroundings, it lashes out, attacking the settlement's mill. When the local watch makes a stand against the Morhell, it is forced to defend itself by attacking anyone who comes close.
This encounter is not tied to any specific campaign or setting and is compatible with all versions of Dungeons and Dragons 5th Edition.
- from the publisher's blurb