Millenniummon

Millenniummon is a Composition Digimon whose name is derived from "Millennium". It fused the overwhelming power possessed by Machinedramon and the data of various powerful Digimon possessed by Kimeramon. It is said to be impossible to defeat, and at present the unclear cause of its fusion is being ascertained, but it is unlikely that any clues will be found.

Digimon Adventure: Anode/Cathode Tamer
Millenniummon was created when a dying Kimeramon fused with a mortally wounded Machinedramon (this Machinedramon was one of the Dark Masters, he was defeated by WarGreymon, but survived) into a single entity. Abnormally strong, he quickly overcomes the DigiDestined as well as the four Digimon Sovereigns. His power was so great, the Digital World itself becomes warped and twisted. Apparently not content to simply defeat them, Millenniummon then imprisons the DigiDestined and revives their previous enemies (Devimon, Etemon, Myotismon and Piedmon) to guard them.

Tai Kamiya's partner Agumon managed to somehow escape, carrying Tai's Digivice. Using it, he desperately called for help. Only one child, Ryo Akiyama, answered the call. Upon his arrival, Ryo thought he was merely having a vivid dream until he was wounded by Kuwagamon. The boy agreed to help rescue the DigiDestined and stop Millenniummon, even though the odds were all but impossible.

For his part, Millenniummon did not bother with Ryo and his growing number of Digimon allies, even as his revived Dark Digimon were being defeated and the DigiDestined rescued. He notes that Ryo appears to have a strange power and was content to simply observe the youth. When Ryo finally manages to reach his castle, Millenniummon appears in his Ultimate form (depending on whether it is Anode Tamer or Cathode Tamer, the player confronts Kimeramon or Machinedramon) before revealing his true form.

After a long and difficult battle, Millenniummon is defeated and Tai, the last of the hostage DigiDestined, is rescued. After being thanked by Tai and a tearful farewell with Agumon and the other Digimon, Ryo returns home.

Digimon Adventure 02: Tag Tamers
In here, Millenniummon appears after Ryo defeats Diaboromon. He splits the Digital World in two with an attack, destroying the mountain they were in and turning Tai and Agumon to stone. He was defeated by Ryo Akiyama once, after which he will create a door which will appear near the town. This door requires six keys, otherwise it will not open. In the other half of the Digital World, before Ken fights Millenniummon as well, an Elecmon will speak of a rumour of a door that leads to Millenniummon's heart, mind and soul (translation has all three as definitions). After that, Ken will defeat Millenniummon as well.

However, Millenniummon has not been defeated yet. Ryo and Ken have to pick up 9 pieces of the Digimental (DigiEgg) of Desire, as those are the keys to the door. After going through the door, they will have to defeat six rounds of enemies in Millenniummon's mind world (as he calls it), before they will face Moon=Millenniummon. Defeating Moon=Millenniummon will turn Tai and Agumon back to normal, but Millenniummon will appear (not Moon=Millenniummon) and launch his Dark Spores at Ryo. Ken intervenes and gets the Dark Spores in the neck instead. Millenniummon will say his power is eternal and disappear.

Wormmon will narrate about how Ken became increasingly cold to him, and hope for the better. Ken, on the other hand, in his Digimon Emperor clothes will stand on a cliff in the Dark Ocean, saying that he is different from Millenniummon, and that he is the Digimon Emperor. This forms Ken's backstory for Digimon Adventure 02.

Attacks
 * Ultimate Fusion (Time Unlimited): Creates a pocket dimension with compressed time, and traps an opponent there for an eternity.
 * Dimension Destroyer: Destroys the pocket dimension it created.

Role in Ultima
In Ultima, Millenniummon is.