Crimson King
The Crimson King is the main villain of Stephen King's Dark Tower series. In the first book of the Dark Tower series, he is briefly mentioned and it is insinuated that he will be the last opponent faced by the hero in his quest for the Dark Tower. The Crimson King first appeared as an actual character in King's novel Insomnia and reappeared in Black House, which was co-written with Peter Straub. His goal is apparently to bring down the Dark Tower, the lynchpin that holds King's multiverse together.
The Crimson King's nature seems contradicted by the various books. In Insomnia and Black House he is portrayed as an omnipotent, omnipresent, god-like force of evil, and seems to be the Satan-like controlling figure over the evil in King's multiverse. He is also revealed to be the true power behind such major evil beings as Atropos and Randall Flagg.
However, when the Crimson King is finally confronted at the very end of the Dark Tower series, he is nothing more than an insane old man locked out on a balcony at the top of the Dark Tower - limited to flinging crude, hand-held seeking grenades (called 'sneetches' here - these are linked in the novel to the 'Golden Snitches' in the Harry Potter stories) at the Gunslinger and his companion Patrick Danville in the series' final battle. There have been some speculation as to why there is such a radical change; one theory suggests that after the events of Black House he lost most of his power. Another theory suggests that the Crimson King is, in fact, the archetype which many of King's other villains, such as It, Randall Flagg, and Mordred Deschain are iterations of in different worlds connected to the Dark Tower. This would mean that several events, such as the endings of Black House, IT, and The Stand, served to weaken the Crimson King by disrupting several of his different incarnations simultaneously.
A "unified" fanon theory exists, suggesting that the Crimson King is a representative (or possibly an avatar) of IT on certain levels of the Tower. Therefore, when IT was defeated the King lost most of his power, resulting in his necessity to act through others (i.e. Atropos in Insomnia, Lord Malshun in Black House, and Flagg). This theory gathers most of its evidence from the King's nature as a were-spider, as well as his mastery of illusions and description of himself as "the Kingfish" (which IT called Itself on occasion).
Another possibility is that the King's weakness in the Dark Tower may represent King's judgement on the nature of evil. Rather than being a seductive or mysterious force, such as it appears in King's other works, evil is finally revealed as being ultimately pathetic.
He was partially erased from existence by Patrick Danville, a character from Insomnia who was foretold to defeat the King, leaving only his floating red eyes behind.
The character's name may have been influenced by the band King Crimson, particularly their first album, In the Court of the Crimson King. It may also have been inspired by the 1895 collection of short horror stories The King in Yellow.