https://www.gamespot.com/articles/more-xbox-one-and-360-games-with-gold-available-no/1100-6455693/

The first batch of December's Games with Gold titles returned to their normal price recently, but a couple of new titles are free right now for Xbox Live Gold members. In addition to the co-op action game Warhammer: End Times - Vermintide, which is available throughout the entire month, Gold members can now pick up a pair of free Xbox One and Xbox 360 games for a limited time.

On Microsoft's newer console, Gold members can now get Back to the Future: The Game - 30th Anniversary Edition for free until January 15. The free Xbox 360 game, meanwhile, is Marlow Briggs and the Mask of Death, which is also available on Xbox One via backwards compatibility. You'll only be able to pick up that title through the end of the month.