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 is the mission halo also know as map (a30) and this modified version simply edits the level to add more enemies and adds reach style models and halo2 & 4 weapons as well as reach, so copy and paste the map file into your halo custom edition "maps" folder and type in devmode console map_name a30, enjoy :D