https://www.gamespot.com/articles/last-chance-for-these-free-xbox-one-and-xbox-360-g/1100-6455676/

It's the middle of the month, and that means now is your last chance to grab some of December's free Games With Gold titles before they return to their normal prices. You have through the end of today, December 15, to pick up episodic adventure game Tales From The Borderlands on Xbox One and the backwards-compatible rhythm game Child of Eden for Xbox 360.

On Xbox One, co-op action game Warhammer: End Times - Vermintide will continue to be free through the end of the month. On December 16, more freebies will become available, including Back to the Future: The Game on Xbox One and Marlow Briggs and the Mask of Death for Xbox 360. That game also plays on Xbox One through backwards compatibility.