PC: Go to the Extras menu and type 20202020 and the word "Nightmare" will change to "20/20/20/20" which will take you to Night 8 once you click it.

Android/iOS: Go to the Extras menu and keep tapping the right arrow until you find Nightmare. Double-tap on him and the word "Nightmare" will change to "20/20/20/20" which will take you to Night 8 once you tap on it.