HALO is one FPS that really did introduce many new gameplay elements. It offers all that is said below and the modifications already in development look to refine and improve the "HALO experience" even more!
This swaps the default Blood Gulch skybox with the one from 'The Pillar of Autumn'. Run Space Gulch.exe to install. NOTE: this is for the original Halo: Combat Evolved NOT Custom edition. Back up before you install.