Throughout this free weekend there will be hundreds of new Official Servers featuring a one-time-only Extinction Event Meteor Impact, and special items to find randomly in supply crates! If you get your hands on one of these Strange Artifacts, you’ll find a steam code on it Careful though, if someone kills you and takes your code, they’ll get the chance to use it first ;)

Source:
http://steamcommunity.com/games/346110/announcements/detail/111816487013236272