Plesiomon is a Mega-level Dragon Digimon, who joined the battle against D-Reaper.
As the D-Reaper prepared to attack the Digital World, Calumon used her light, and led the Digimons to evolve to the Mega level to fight the D-Reaper. And as Plesiomon evolved from the previous level to the Mega level, it joined with other Digimons alongside the Sovereign to fight D-Reaper.