Moon=Millenniummon (Adventure:)

Millenniummon is a fictional character and Mega-level Digimon from the series Digimon Adventure:. It serves as the main antagonist of the anime. Millenniummon was the weapon of the army of evil Digimon who participated in the war between light and dark Digimon.

Story
Before the DigiDestined arrived in the Digital World, Millenniummon was a pure destructive force that was responsible for the deaths of countless Digimon during the ancient war between good and evil Digimon. It served as a weapon for the Dark Digimon Army. However, they were unable to control its power, forcing WarGreymon, MetalGarurumon, Hououmon, Rosemon, Vikemon, HerculesKabuterimon, Seraphimon, and Ophanimon to sacrifice themselves in order to defeat it.

Moon=Millenniummmon
Moon=Millenniummon is one of Millenniummon's Mega forms.

After Millenniummon (as ZeedMillenniummon) was defeated in the war by WarGreymon, MetalGarurumon, Hououmon, Rosemon, Vikemon, HerculesKabuterimon, Seraphimon, and Ophanimon, shards of its body fell back to the Digital World. One of these shards became Moon=Millenniummon.

Moon=Millenniummon devoured data from the real world and discharged it as black lightning to the Eternal Continent, where it could be used to restore Millenniummon's body and digivolve its servants into more powerful forms. For example, Devimon was able to use Moon=Millenniummon as a catalyst to digivolve SkullKnightmon into AxeKnightmon. Devimon also used Moon=Millenniummon to digivolve himself into NeoDevimon, and AxeKnightmon used it to digivolve NeoDevimon into DoneDevimon. Moon=Millenniummon was also revealed to be one who digivolved Orochimon into Nidhoggmon during the battle of simulated Tokyo.

ZeedMillenniummon
ZeedMillenniummon is one of Millenniummon's Mega forms.

In the past, ZeedMillenniummon is the weapon of the army of evil Digimon, but they unable to control it due to immense power. However, WarGreymon, MetalGarurumon, Hououmon, Rosemon, Vikemon, HerculesKabuterimon, Seraphimon, and Ophanimon defeated it by sacrificing themselves.