Yami: Why would you want to do that?
Anubis: What?
Yami: What's the point in destroying the world? What do you gain from it?
Anubis: ... I don't understand the question.
The Generic Doomsday Villain is an overpowering antagonist with no clear motive or plan behind their actions. They're one of the most basic story villains possible. The reason that they're so generic is that they are a Flat Character with a minimum of characterization. Their Establishing Character Moment is generally the most development that they get, designed to show the audience what sort of threat that they're facing. The character may or may not provide an explanation of their Evil Plan, but their overall goal is some sort of Apocalypse How, providing the "doomsday". At the very least, they plan on destroying the people within the town/country that the heroes are from.
When part of a recurring work, this character is forced into one of two outcomes: they either remain a Static Character who has almost nothing aside from "I want to destroy everything", or they become a Dynamic Character and lose their "generic" quality. Some writers intentionally make these characters flat to highlight their unhealthy psyche.
This type of villain is especially common for video games, as they exist more as obstacles for the players to defeat than characters to interact with.
Contrast with Complete Monster, who may not have a plan at all, but does have plenty of characterization and an established motivation, and Feral Villain, where the character has no sentience with which to have characterization.
