The episode title roughly translates to "monsters".
Keeping up with his theme of Keanu Reeves' costumes, Max dresses as Chevalier Danceny from the movie Dangerous Liaisons (1988).
During the party scene where Danny is looking for Grace, the song that is playing is Levels, by Nick Jonas, who plays hacker Ian Wright in season 4 and 5.
This episode is the now traditional annual Halloween themed episode, usually set on the 30th-31st of October every year.
The interior of the cottage (or at least its kitchen) is the same as the house of the killer's grandma in season 3 episode 5.