Overarching Villain

4 vs. Satan:
*Satan.
Alex in Princess Quest:
*Malthor van Dracula
The Stone of Power:
*Original Four-Parter: Artimus. He's the Big Bad of the first game and behind the second and fourth games' main villains' Start of Darkness.
The Three Kingdoms
*Demon Lord Degares.
Dragondawn
*The three dragons.
Harter Knall
*King Greyland.
Hero Story
*Shevu.
Kill the Demons
*Casper and Rustoc.
Satan Saga
*Satan, then Pope Bonmos, then King Greynor.
Story of Hero:
*Elizon Tataros.
The Legend of Devils
*Satan
The Sidestory Duology:
*Ravydion. It's implied Rudolf Hitler used to be his underling.
The Time Of Bloody Fear:
*Maltros is one for the entire franchise. He's also behind the second game's main villain Start of Darkness and even the demons worship this man for his actions.