The ability to create and tinker with custom maps in the Halo series was first introduced in Halo 3's Forge mode. Players went wild with it, creating all sorts of maps and introducing new gameplay rules to go along with it.
Take, for instance, Duck Hunt, where the objective isn't exactly a team versus team scenario, but rather one sniper against many "ducks" who try to scurry safely across the map.
Here's an example of how that works out:
Forge maps can be fun and creative twists on the standard shooter modes with standard rules.
Now that Halo 3 is back on the mind with the release of Halo: The Master Chief Collection, player GrammyXBL has been hard at work bringing back a handful of those classic custom Halo 3 maps in brand new HD.
You can see a montage of a preview of some of those maps in the video at the top. GrammyXBL certainly isn't the only one remaking some of these custom maps/games, but the creator in this case seems to have worked on quite a few and is offering to share them with other players.