Apocalymon

Apocalymon is arguably one of the strongest villains in the Digimon series, a human-like being who is connected to a large structure. Filled with dark thoughts, Apocalymon was born from the assimilated data of every Digimon that was "destroyed" by the Digital World's evolution. Apocalymon's arrival was written down in ancient prophecies and the hatred of the Digimon that formed him lingered on. A sadistic & tormented soul, Apocalymon wanted to make the Digital World a living hell to justify the darkness that composes him. He knows all the evolutions and attacks of Digimon.

Attacks

 * Grand Death Big Bang (Total Annihilation): Suicide attack that produces an explosion strong enough to wipe out both the Real and Digital Worlds.
 * Death Evolution (Reverse Digivolve): Reverts Digimon.
 * Darkness Zone
 * Virus Grenades
 * Copied attacks: The giant claws morph to enable Apocalymon to use the attacks of others.
 * Mugen Cannon (Giga Cannon): Machinedramon's attack
 * Ulitmate Stream (River of Power): MetalSeadramon's attack.
 * Bloody Stream (Crimson Lightning): Myotismon's attack.
 * Death Claw (Touch Of Evil): Devimon's attack.
 * Digi bomb (datamon)

Digimon Adventure
Apocalymon appeared to destroy the DigiDestined with his powers. He destroyed the Tags and Crests and sent the DigiDestined to the Data World before they returned using the "Crest" power that was within them all along. It took a combined force of WarGreymon, MetalGarurumon, Garudamon, MegaKabuterimon, Lillymon, Zudomon, MagnaAngemon and Angewomon to defeat him. But when he tried to commit suicide/genocide with his self-destruct Total Annilation, the DigiDestined sealed him with their Digivices so that he would only kill himself.

Digimon Tamers
What might be Apocalymon was among the shadowed Digimon that wanted Rika to tame one of them.

The Adventurer's Battle
Apocalymon attempted to cross into the Real World to destroy it until he was confronted by Omnimon. But when he was seemingly killed by Omnimon, Apocalymon's data reformed into a new evil to carry out his justice. Its name was Mephistomon.