Lucemon

Lucemon was a Digimon who believed others could not think for themselves, and that the world would be a better place if he had absolute power and made all the decisions. Lucemon appeared during the war between Human-type and Beast-type Digimon that occurred in the ancient past of the Digimon Frontier timeline. Lucemon was able to stop the war, and created the rules for a peaceful society where he had been the ultimate sovereign. However, he was corrupted by his own power and became oppressive towards all other Digimon.

His power made him unbeatable until the first Legendary Warriors appeared and stopped him by sealing him inside the Dark Area. The governing of the Digital World was handed over to the three Celestial Digimon - Ophanimon, Seraphimon and Cherubimon. Lucemon was able to corrupt Cherubimon, and had him and, later, the Royal Knights collect the data that would be needed to revive him. They were successful, and Lucemon was reawakened. While he never did any fighting on screen in his original form, while he was imprisoned he was able to send out a burst of power that defeated a relativity unhurt EmperorGreymon and MagnaGarurumon and blast them away with his Grand Cross attack when he appeared on the moon.
Lucemon Chaos Mode is Lucemon's ultimate form. In this form, Lucemon is now an adult-like figure with a white-angel/black-demon 2-toned attire - representing Lucemon's ability to combine the powers of light and darkness.

(Source: Wikipedia)