Prey (2006)

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 Studios under the direction of 3D Realms.