Doomsday is the second digimon emporer, he is much more powerful and evil then the first.
Doomsday was once a human child but when he was very young he was sucked into the digital world. He grew up in this strange world with no friends and no family making him an easy target for evil digimon. But nobody could have possibly immagined that this small child had such great power untill Myotismon found him. When Doomsday was 9 years old Myotismon was defeated and Doomsday was alone again he vowed to avenge Myotismon who was like a father to him. Doomsday is next heard of after King Drasil returns in the upcoming seventh series Marcus Daemon defeats Drasil but then he hears Doomsdays voice claiming to be the all powerful master of the digital world called "The Masked King". When Marcus challenges him he appears as a tall devi mon shaped shadow but that is just his first digimon.
During the fight with Marcus, Doomsday uses a lot of different digimon:IceDevimon Piedmon BlackwarGreymonX(one of doomsdays actual digimon) Beelzemon(doomsdays digimon partner it fights along side BlackwarGreymonX)
When beelzemon is injured Doomsday appears and attacks Marcus' ShineGreymon Doomsday also has digimon like powers his attacks are: Pandemonium Shocker: He shoots red lightning from his fingers Chaos strike: he uses dark energy and his fist glows black then he punches his opponent (it is actually his DNA charge) Demon Inferno: he uses all his anger to make a huge blast of dark energy
As a Digidestined
After being defeated by Marcus, Doomsday becomes human again but he has no name except doomsday so he takes that as his last name and finds out that his father was called Maxx so he takes his fathers first name and becomes known as Maxx Doomsday. He helps D.A.T.S to defeated evil digimon and in the process he regains his digimon and gets a new one Dexmon.At the end of the seventh series he finds out his father created an all powerful dark digimon.
With Marcus, Thomas and Yoshino they defeat his father and his digimon Arkadimon