From the moment the D-Reaper was defeated. Calumon felt different, he knew something was off but was too happy to finally be a real digimon he didnt care. The carefree funloving digimon went on about his new life. Looking for ways to help other digimon. After sometime Calumon felt a power within him. Something strong... He thought that his Evolution Matrix had returned so he went to Azulongmon for help. Azulongmon mistakingly thought that Calumon may be on the verge of digivolving. He was correct and WRONG at the same time. A portion of the D-reaper resided within Calumon feeding on his digital energy his very life force. Being as active as he was Calumon never felt weakened by this.
The form inside him grew stronger. One day Calumon woke up and was not able to move, but his body was moving on its own... The lifeform within him had taken control and soon would take him over forcibly digivolving him to a new mutant virus. The lifeform became aware and called itself ReaperCalumon.
This new Reaper started to take over the digital world. This was the start of 'The Event", Guilmon and his friends tried to contact the real world but the new Reaper locked the real off from the digital world. Within the reaper Calumon was still alive and used his last bit of power to summon the tamers he once knew to help him and save the digital world.
Reaper Calumon is not truly a digimon and more like a virus, he has no real attacks or power level but his power does rival that of Mega level digimon.