The undead are relentless in their thirst for human flesh, especially the ones in Treyarch’s Call of Duty games. As some of you may have already discovered, Black Ops 3’s Zombies can’t be contained to just one mode.

“It’s part of our studio’s DNA to launch a game with a few surprises – and we see that some of you have already begun to find your way to this one,” said Treyarch in a note to fans, referring to a secret game mode that wasn’t previously advertised. “If you complete the Campaign, you can access ‘Nightmares’ mode and play it again in an entirely new way…but with Zombies.”

Pretty straight forward. Complete the campaign, kill even more zombies. Have any of you unlocked it yet? Check out the new trailer above, set to the hard rock sounds of Avenged Sevenfold.

While you’re at it, here are 9 other all-new things you need to try in Call of Duty: Black Ops 3.