https://www.gamespot.com/articles/xbox-one-games-with-gold-september-2018-are-now-av/1100-6461519/

It's September, so Xbox Live Gold subscribers can download a new batch of Games with Gold. The newly available games are Prison Architect for Xbox One and Lego Star Wars III: The Clone Wars for Xbox 360 (also playable on Xbox One). The former is free through the end of September, while the latter is free between now and September 15.

As you can probably tell from the title, Prison Architect puts you in charge of a prison. You get to design the layout of the building and manage how the occupants are treated. You can aim to make your prison a pleasant place designed to rehabilitate the misguided criminals, or you can give into your darker urges and inspire things like arson, murder, and riots.