Belphemon was an evil king digimon whose Data had to be put into a sleep state by the digital world's system. He was a demon lord, a group of beings who almost brought ruin to the Digital World. However, they were stopped and reverted to Digi-Egg form, then sealed away to be forgotten. A scientist named Akihiro Kurata discovered a temple which hid the sealed Digi-Egg of the Demon Lord of Sloth, Belphemon. He retrieved the egg and it eventually hatched into Belphemon Sleep Mode, after which Kurata began a campaign to awaken the Great Demon Lord by collecting "Digimon life energy", which would allow him to conquer both the human and digital worlds. Using his army of Gizumon to permanently murder many different Digimon, Kurata eventually collects the required energy after slaying the massive ElDradimon. Believing he has successfully brought Thomas H. Norstein to his cause, Kurata uses the "Digimon control device" Thomas had made for him to command Belphemon. However, his plan didn't go as he though, and the control device was destroyed, leaving Belphemon to eventually awaken out of his sleep mode, and wreak havoc upon the city in his Rage Mode. It took the combined forces of several heroes to seal him away once more.
It is said that Belphemon Sleep Mode awakens from its eternal slumber once in a thousand years. When it awakens, it regains its natural shape and changes into an incarnation of rage, and anything that enters its field of vision becomes a target for destruction. It is said that just by suffering Belphemon Rage Mode's roar, Ultimate Digimon and below will have their data disintegrated and will instantly die, and not even Mega Digimon can escape unharmed. His rage mode led him to be able to rule a majority of the Dark area before his first defeat. Belphemon appears to be a being purely composed of Sloth and Rage due to these circumstances. During Sleep Mode, he seems to float aimlessly, wreaking havoc just by snoring, but still responds to the world around him, as if he was were sleepwalking. During Rage Mode, he wreaks havoc for seemingly no reason, in a fit of blind fury. This isn't to say he is a mindless beast, though, as he can (and often will) work with others to get what he wants (albeit he shows hints of laziness during this), and can even rule amongst the other Demon Lords without issue.
- Eternal Nightmare: Invokes a nightmare with its peaceful snoring. The force of the snores can also level buildings as if it were a beam attack.
- Lampranthus: Casts black flames from the chains coiled around its body. The flames snake around an enemy, burning them.
- Flight: Belphemon Sleep Mode is able to float freely.
- Physical Durability: He does not seem to flinch much at all even when hit with powerful attacks.
- Lampranthus: Same as Sleep Mode, except more powerful.
- Gift of Darkness: Unleashes a slash attack from its claws, which are said to be clad in the flames of hell.
- Dark Horn: He attacks with one of his horns, coated in dark energy.
- Lightning Horn: He attacks with one of his horns, coated in Electrical energy. Can be used in conjunction with the above.
- Physical Durability: While his sleep mode has far mroe durabiltiy at the cost of attack power, Rage Mode is no joke.