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!
Heretic level is the mission halo but you play as the covenant and try to survive the heretic hordes looking for you while you try to rescue survivors and escape, place the map file in you halo custom edition "maps" folder and type in halo devmode console map_name a30, enjoy :D