This is one hell of an Easter egg.

Treyarch, the developer of Call of Duty: Black Ops III, has put a secret mode in the latest first-person shooter game. Treyarch has been putting a zombie co-op play mode in its Call of Duty games for a while. Zombies even make an appearance in the single-player campaign of Black Ops III.

But if you play through the entire campaign mode, you get a special Easter egg, or hidden inside joke, as a gift from the developers at Treyarch. It comes after the credits finish. If you wait long enough, you’ll see that in the main menu, you can unlock something called Nightmares mode. You can fast-forward the credits by moving the analog┬ástick.

In the Nightmares mode, you can play the entire single-player campaign over again, but this time with different characters and a different narrative. The characters are zombies, and it’s taking place in the zombie apocalypse universe. If you really like single-player campaigns, this is a pretty huge treat, and it is separate from the zombies mode. Activision’s Call of Duty: Black Ops III debuts on Friday on the PlayStation 4, Xbox One, and PC.