The Bioshock series from Irrational Games and 2K is one of the deepest darkest worlds you can find on consoles, throwing you into an alternative history and seeing you dealing with the consequences of some really screwed up stuff.
And things don't get much more screwed up than Bioshock Infinite, with a brain twisting final act that I'm pretty sure I still don't understand. If you've never had the chance to visit the floating metropolis of Columbia or you want to get behind Booker DeWitt's guns again, there's never been a better chance as the game is currently free on Xbox Live Gold. It's a terrifically fun shooter with plenty of inventive death-dealing, especially when you combine your guns with the increasingly over the top powers you'll gain as you progress.
You'll also get to interact with Elizabeth, a NPC companion who bucks the trend by not only being well-written and engaging but also genuinely useful in a fight thanks to her special abilities. It also helps that she's generally invulnerable. Did we also mention that the game looks absolutely stunning?
Head to Xbox Live now and grab Bioshock Infinite for free on Xbox 360. And if you've got an Xbox One you'd be crazy to miss Rayman Legends, which is also free at the moment.