DayZ is an open world survival horror video game mod for the 2009 computer game ARMA II and its 2010 expansion pack, ARMA 2: Operation Arrowhead. It has been widely praised in the game media for its innovative design elements, with Kotaku and Eurogamer describing it as possibly the best zombie game ever made, and PCGamer saying it was one of the most important things to happen to PC Gaming in 2012. The mod was responsible for putting the three year old game into the top seller charts on Steam for over seven weeks, spending much of this time the top selling game, and is responsible for over 300,000 unit sales within two months of its release.