Vademon is an Ultimate Digimon.
In Adventure, Vademon is a merchant who sells "hearts" and had a deal with DemiDevimon. Vademon battled Izzy and Tentomon his dimension and steals his Crest of Knowledge, thus resulted Tentomon devolved into his fresh stage. Vademon was finally defeated by the newly-evolved MegaKabuterimon later on.
In Frontier, he is a fortune teller in the Akiba Market.
In Adventure: many Vademon seems to be controlled by Millenniummon. They successfully revives Millenniummon by using the data collected from Sakkakumon, and then they were killed after the revival of Millenniummon.