Halo Zero Level Editor
From Halopedia, the Halo wiki
Summary
The current version of the Halo Zero Editor is 0.3 and has only been available very recently. It has been announced that the current Editor will be updated soon.
Also on that note, when the Editor is downloaded, it automatically upgrades your Halo Zero to the newest version available only along with the download Version 1.8.6.2.
The current version of the level editor is quite unstable and you have to back the levels up quite often.
Mods
The Level Editor allows players, mainly the members of the Halo Zero Community, to mod maps.