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 02: Tag Tamers
Millenniummon digivolves from a Megadramon in line 33.

Digimon Adventure 02: D-1 Tamers
It is possible to obtain a Millenniumon by fusing a Machinedramon and a Kimeramon and obtaining 96 jogress points.

Digimon World Dawn/Dusk
You get Millenniumon by DNA digivolving Kimeramon and Machinedramon with 27000 Dark EXP, 320+ Attack, and Lvl. 55.

D-Ark 1.0
Millenniummon is a wild Digimon with 15 HP and 4 AP. It is #49 in the D-Ark Notes, and its codes are -576 and -696. It appears as the boss of Secret S.

D-Tector 2.0
In the Japanese release of D-Tector 2.0, Millenniummon can be obtained by scanning.

DigiWindow
Millenniummon appears in the sixth Northwest area of Dark Castle and can be unlocked by winning the "Skull" mini-game.

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.