D-Reaper

D-Reaper was nigh-unstoppable force existing only to destroy anything that goes beyond it's bountries.

Digimon Tamers
The program came to be in 1970, as “the reaper” program created by the American Department of Defence to delete excess data & manage the growth of artificial intelligence. The reaper’s first job was to delete “the worm,” the very first computer virus. In the 80's, the reaper was used as the basis for the security system of “Project Echelon”. In 1990, the reaper attacked the fledgling Digital World, which had recently become populated by the Digimon. The Reaper went on to delete Digimon, absorbing their data, & underwent a evolution of it's own. It buried itself away, beneath the Digital World, and slumbered as it transformed, into a being of sentience and great power.

By the time of the Tamers series, the newly created D-Reaper bgan to stirr from the sheer amount of excess data that had been caused by Digivolving and the growth of the Digital World during it’s period of inactivity. It took Juri Katou to learn more about humans, creating it's first Agent. The main part of D-Reaper followed the Tamers to the real world through the Hypnos system, and immediately began to spread over Shinjuku, keeping the Kernel Sphere high between the two towers of the Hypnos building, with Juri sealed inside as it's source of energy due to her grief. Only the BioMerged Digimon were able to fight it, until it evolved in the Mother D-Reaper. But it thanks to the combined effort of everyone, & Operation Doogle Bug, that the D-Reaper was regressed back into it's original, harmless state.