|Human Partner||Akiyama Ryo|
|Voice Actor||Egawa Hisao (江川 央生)|
Millenniumon (with Chimairamon)
At some point between the events of "The Chase! Hurry to Japan" and "The Four Rulers of the Demonic Mountain! The Dark Masters!", while the Chosen Children are in the real world fighting Vamdemon, Mugendramon and the other Dark Masters conquer and reformat the Digital World into a shape that makes it easier to rule: Spiral Mountain. He rules Spiral Mountain's urban area and commands the Metal Empire army. According to Gennai in The Holy Swordsman! Holy Angemon", he (like the other Dark Masters) draws his power from the distortions created by the existence of Apocalymon.
When the Chosen Children return to the reformatted Digital World in "The Four Heavenly Kings of the Demonic Mountain! The Dark Masters!", the Dark Masters attack them. Mugendramon is the second to attack, easily overwhelming the Perfect-level partner Digimon with a Mugen Cannon barrage and causing the ground the cave in, sending them to face Pinochimon. The four come together to wipe the Chosen Children out, but are foiled when Piccolomon intervenes to hold the Dark Masters off and send the Chosen Children far out of their reach. The four kill Piccolomon.
In "The Raging King of the Seas! Metal Seadramon", Mugendramon is with the other Dark Masters in their castle at the top of Spiral Mountain, where they are observing the Chosen Children. He, Piemon, and Pinochimon are still there in "Silence at the Bottom of the Ocean, Whamon", witnessing the defeat of Metal Seadramon and the release of the ocean area from Spiral Mountain.
In Bombing Mission! Mugendramon, Mugendramon sends the Metal Empire after Yagami Taichi, Yagami Hikari, Takaishi Takeru, Takenouchi Sora, and Izumi Koushiro when they enter the city in search of medicine for Hikari. He and his Hagurumon track Taichi and Koushirou through the city by tracing Koushirou's internet use, but when Koushiro catches on and tricks the system into thinking he is logging on in hundreds of places in the city at once, Mugendramon orders the execution of "Plan Z" to destroy the entire city and smoke the group out. When Taichi and Koushiro reunite with the others with the medicine, Mugendramon bursts out of the ground and confronts them personally. His Mugen Cannon levels a skyscraper and tears the ground open, sending them falling into the underground sewer system.
In Farewell Numemon, Waru Monzaemon contacts Mugendramon to report that he has fought, and been defeated by, the Chosen Children. For his failure to first report that he knew their location, Mugendramon kills him by detonating his control room, then begins his own pursuit of the group. He tracks down Taichi, Koushiro, and Andromon, who soon reunite with Sora, Takeru, and Hikari. He overpowers all of them and effortlessly slaughters a horde of Numemon that attacks him in an attempt to protect Hikari. Hikari's grief at witnessing this triggers her powers, which restore Agumon's strength and evolve him into War Greymon, who slices Mugendramon into pieces with his Dramon Killers. His defeat causes the collapse of the Spiral Mountain city area and returns the land to its rightful place.
Mugendramon and the Dark Masters appear in a flashback in the prologue of "Loss", which depicts their initial battle with Himekawa Maki's original group of Chosen Children. The Dark Masters initially easily overwhelm the Perfect-level team, but when Homeostasis triggers most of the team's evolution into the Four Holy Beasts, they are defeated.
Mugendramon serves as the second to final boss of the game in Cathode Tamer. Upon being defeated by Akiyama Ryo, he becomes Millenniumon.