Prey (2006) Steam Gift

Aliens turned Tommy's world upside-down. Now he's about to return the favor. In Prey, players enter a living spaceship which enslaves alien races and devours humans for lunch. Prey turns the first person shooter genre upside-down with awesome new gameplay features like wall-walking and gravity flipping, making for intense single-player and multi-player experiences. Prey is built on a heavily modded version of the Doom 3 engine and is developed by critically acclaimed developer Human Head Stud
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