The ChronoCore is a digital lifeform which serves as the main antagonist in Digimon World Dawn and Dusk. It is a sapient virus which degrades and destroys all data in the Digital World. It improved itself with the Digi-Entelechia produced by Calumon, then set out to take down Yggdrasill and rewrite the Digital World, in order to make the Digital World itself perish.
Digimon World Dawn and Dusk
The YMIR Project was started by CITY long ago in order to set a lifespan on Digimon in order to make them evolve like humans. Although the project was canceled, it produced the "Chrono Data" in the form of the Super-Ultimate Digimon, Chronomon Holy Mode. When Chronomon was forced into its destructive DM form, the "ChronoCore" virus was produced. This virus had the end-goal of taking down Yggdrasill and rewriting the Digital World.
After Chronomon DM is defeated, the ChronoCore fuses with Grimmon, giving it the "Chrono Destruction" (Chrono DSR) ability, in order to take down CITY and the rest of the Digital World. Grimmon first uses "Chrono Destruction" to steal all of the Digi-Entelechia from the CITY Digimon, reverting them to Digi-Eggs. These Digi-Entelechia increased its power and gave it the ability to "control all Digivolution here in the Digital World." Grimmon is driven off by the protagonist, but after being fought again at Limit Valley, it appears at Access Glacier, where it begins using the ChronoCore to possess the humans of CITY. Although the protagonist frees their own team, the ChronoCore is still in control of the other team and it combines with that team leader's partner to form either ChaosGallantmon C (Dawn) or Ophanimon C (Dusk). The protagonist ventures into the other team's domain in order to free them of the ChronoCore's control, but after doing so, the ChronoCore breaks into the Chaos Brain and forces Grimmon to evolve into ChaosGrimmon and then ExoGrimmon, which it dubs the "ultimate Chrono DSR". However, Grimmon's power is not enough to prevent defeat, and the ChronoCore fades away, lamenting Chronomon's absence.