The Original Villains- Season 1:

Digivolution: Demidevimon > Devimon > Myotismon > Venommyotismon > Malomyotismon
The original villain of the show, Devimon sought to rule all of File Island. He attempted to gain control of good digimon through the use of Black Gears. Once these gears found their way through a Digimon's exterior, they would disrupt their normal programming and make them turn evil. He is a tall, dark creature with tattery wings and long hands. With his Touch of Evil, he can totally control any being. Devimon swore to rid the world of the DigiDestined, his only threat, and take over the entire Digital World. After taking control of Leomon, he reigned his power over the Digital World. Destroying villages and conrolling digimon.He employed Ogremon in his services and Leomon by controlling him through a Black Gear.
The kids eventually defeated him when Patamon Digivolved to Angemon in Episode 13: The Legend of the DigiDestined

Digivolution: Demidevimon > Devimon > Myotismon > Venommyotismon > Malomyotismon
DemiDevimon is Myotismon's lackey. He is a trickster and a liar, attempting to deceive the kids whenever possible. Although a force of evil, he knows to run and cower when he is outmatched. He caused a lot of trouble for the Digidestined, such as making Sora think her crest wouldn't work, getting Joe lost, and making T.K. think Matt didn't like him anymore.
Demidevimon is very loyal to Myotismon. He would do anything Myotismon would tell him and even helped him when things seemed lost. In the episode "The Prophesy", Myotismon digivolved into Venom-Myotismon and needed energy (food) so Demidevimon let himself be eaten by Venom-Myotismon.

Digivolution: Demidevimon > Devimon > Myotismon > Venommyotismon > Malomyotismon
Myotismon is a Digimon of the vampire nature. He has large forces that help him to try to take over the real world. He has an entire army of different henchmen and a variety of different attacks. His Crimson Lightning is a bust of red discharge. His Grizzly Wing unleashes a swarm of bats, and his Nightmare Claw will knock you unconcious. Under his employ, he uses DemiDevimon to distract and slow down the Digidestined while Gatomon sought out the biggest, baddest Digimon she could find for Myotismon to take through the portal. Myotismon maybe an ultimate level digimon, but even the combined powers of half the Destined can hardly beat him. Even if they do get close, he is smart and flees from battle. He was later defeated in the real world when Gatomon Digivolved to Angewomon and used a Celestial Arrow to pierce his evil heart. However, unknown to the kids, that wasn't the end of Myotismon. He was reborn very shortly thereafter as VenomMyotismon.