From Halopedia, the Halo wiki
alphamoon is the working title of a cut campaign level produced during the development of Halo 2. The level was set on Basis, the moon of the gas giant Threshold - the planet around which Installation 04 orbited. The level would have seen the player play as the Arbiter in an assault against a heretic camp on the moon.
alphamoon was first revealed in a stream of the Halo 2 E3 demo, shown by Frank O'Connor and Brian Jarrard following Jarrard's chance finding of a Halo 2-era Xbox devkit with the build on it in his home. alphamoon is one of a number of levels included in this build. The level as presented in the stream is based in the Halo: Combat Evolved game engine, using the Combat Evolved HUD, plasma rifle, and plasma pistol - though the Unggoy and Sangheili art assets featured are their Halo 2 incarnations. The level is from early Halo 2 development and is not optimised, resulting in a very low framerate while playing.
In the level dialogue, the Arbiter is referred to by the title of "Dervish". This name was the original title for the Arbiter in Halo 2 development, but was ultimately changed to avoid being unintentionally offensive to Muslims.
The level is set in a heretic camp on the moon of Basis. While this concept did not make its way into the final Halo 2 campaign, this idea is the basis for the setting of the multiplayer level Burial Mounds.