Long before the Digital World's existence began in 1973, there used to exist another Digital World, one which was terrorized by an entity of pure fire incarnate; AncientGreymon. It was the living embodiment of everything that is Fire Element, and as such, is not only the most powerful Fire Attribute Digimon, but this also makes it the most powerful species of Greymon, even to the point of surpassing the later seen DeusGreymon in power. The reason as to how? DeusGreymon is unable to enter a rare state known as "Overclock", which allows the user to run at power levels beyond what they're limited to. No modern day Digimon is born with this ability, nor can any of them actually acquire it whatsoever. The only Digimon that have access to this ability are strictly the Ten Legendary Warriors in their own Ancient forms, not any of their weaker forms. When entering Overclock, AncientGreymon melts the entire landscape around it into a molten inferno, basically turning the surrounding area into a fiery corona. Compared to the other nine Legendary Warriors, AncientGreymon is almost always in it's Overclock state, and nothing, and literally nothing, will want to ever face AncientGreymon in it's limitless powered state. It ain't called "The Destroyer of Worlds" without a good reason, for it's the very cause of the destruction of the ancient Digital World, because it grew far too powerful with it's Overclock that it essentially incinerated the entire plain of existence.
Tokutaro "Taro" Kobayashi
Programmed into the Prototype Digivice by an unknown entity, AncientGreymon is forced into a dormant state, from which it slowly begins to free itself by essentially taking over Taro's mind bit by bit. AncientGreymon's personality slowly begins to turn Taro into a harsh, aggressive, and easily aggravated person, all the way until Taro completely goes berserk and transforms into AncientGreymon, finally freeing the ancient Digimon from it's dormant state. After a fierce, yet one-sided, battle with a mysterious entity, AncientGreymon goes back to being dormant due to Taro's mind taking back over, but now that nothing is truly restraining it anymore, AncientGreymon can potentially awaken yet again at any time, and the fact one of the Golden Guardians knows who he is doesn't help matters...