https://www.polygon.com/2017/11/22/16691928/xbox-games-with-gold-free-games-december-2017

Xbox Live’s Games With Gold series will close out 2017 with the multiplayer team-up action of Warhammer: End Times — Vermintide and Telltale Games’ sequel to the Back to the Future trilogy.

Xbox One gets Warhammer: End Times — Vermintide for the whole month. From Dec. 16 to Jan. 15, Xbox Live Gold subscribers get Telltale’s Back to the Future: The Game — 30th Anniversary Edition, comprising all five episodes of the series that originally launched in 2010 and 2011.