December's Xbox Games with Gold lineup includes original Xbox backwards compatible title Mercenaries.
Mercenaries, developed by shuttered studio Pandemic and published by LucasArts, launched in 2005 on Xbox and PlayStation 2 and became something of a cult hit. It had a vast sandbox and an impressive destruction system that meant you could level pretty much all the buildings in the world.
Mercenaries counts as the second Xbox 360 title for December's Games with Gold lineup. You can download it from 16th December to 31st December. The other Xbox 360 title is BioWare's Dragon Age 2, which you can download from 1st December to 15th December. Both games are playable on Xbox One via backwards compatibility.
On Xbox One, puzzle game Q.U.B.E. 2 is available to download during December. Never Alone is available to download from 16th December to 15th January 2019.