Saber Interactive has released a free update for World War Z that adds a new difficulty option for its existing chapters.

The cooperative shooter came out for PlayStation 4, Xbox One, and PC on April 16 and sold 1 million copies in its first week. This update, called Six Skulls, will give fans an excuse to jump back into the game and try the more challenging versions of levels they’ve already beaten.

The next free downloadable content will come with (the not-so-subtle name) The Big Update. It will add weekly challenges, characters skins, and weapon variants.

World War Z takes place in the same fictional universe as the 2013 film of the same name, which was based on the 2006 book. Zombies have long been a mainstay in gaming, especially in the cooperative shooter scene following the 2008 release of Left 4 Dead.