A Halo spawns in the map, and whoever picks it up becomes the Saint. The Saint gets a bonus of health/armor and kills all the Sinners: all the other people in the game. If a Sinner kills too many fellow Sinners, he becomes a Heretic and can only try and fart people to death. It's very entertaining. If you're looking for a good laugh, get this mod now.
NOTE: Unzip into your Quake 3 directory, not BASEQ3!