Complete Monster: Randall Flagg is an Evil Sorcerer in the medieval country of Delain. Here, Flagg has Queen Sasha murdered, poisons King Roland, frames Prince Peter for the crime, and then plunges Delain into a new Dark Age by manipulating the remaining heir, Thomas.
Generic Doomsday Villain: Flagg's ultimate goal, apparently, is not to rule the kingdom, but to plunge it into chaos. What he stands to gain from this is never explained.
Aside from it being heavily implied in-universe that Flagg is a demon, multiple explanations can also be found in the other stories in which Flagg appears. Flagg, whether as the personification of evil or as an emissary of the Red, simply hates things like light and order and seeks to destroy them at all costs. The last book in the Dark Tower series, along with some supplementary comics, explains that Flagg was actually born in Delain as the peasant bastard son of the wizard Merlin, had always grown up with the chaos of the Prim haunting his mind, and just for good measure has a bit of Rape as Backstory tinting his past.
Genius Bonus: Lucifer derives from the Latin lucem ferre, meaning "lightbringer". As in, Thomas the Lightbringer.
Lighter and Softer: By Stephen King standards, anyway. It's still pretty dark, but it's written for a slightly younger audience than King's normal work.