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.

Some apparently contradictory features of the Crimson King are mentioned in various books. In Insomnia and Black House, he is portrayed as an omnipotent, omnipresent, god-like force of evil, a Satanic figure creating the evil in King's multiverse. He is also revealed to be the true power behind such major villains 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. The character was received a particular letdown by many fans due to the fact that the majority of his final conversation with Roland consists of high-pitched, guttural shouts of "EEEE!" Some readers felt it difficult to resolve the incongruity between the Gunslingers' greatest foe having such a strong trait reminiscent of mental retardation.

There has 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 he is, in fact, the archetype which many of King's other villains, such as It, 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 suggests 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, forcing him to act through others (i.e. Atropos, Lord Malshun, 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.

Ironically, Flagg's appearance is not described as threatening. He appears to be merely an average-looking man, taking on the appearance of whatever the local people look like. However, his attire is strictly Americana. He dresses in typical American clothing such as blue jeans, a hooded sweater, and boots with worn down heels. Particularly noteworthy are the buttons he collects and attaches to his clothing over the course of his appearances. Amongst these are a peace symbol, a smiley face, and a "CK" button (which most likely stands for Crimson King, though appears similar to the logo for Calvin Klein). In the Dark Tower series, he also has one that has a picture of an eye on it.

Recently King revealed Flagg to be a guise of the Man in Black from his Dark Tower series. So whilst the name Randall Flagg is more popular with the fans and has been used more, it would be more accurate to refer to his character as his true name, Walter o'Dim.

The King, Flagg, and It are all obsessed with creating heirs, a way of continuing their legacies. All of their children are treated with a reverence and fear like unto that of the anti-Christ, although Mordred is the only one to have ever been born.

Mordred Deschain and It share similar "true" forms, in that both of them are actually hideous, spider-like entities. Both are capable of metamorphasizing into different shapes, though this is far more pronounced in It than in Mordred. Mordred and Flagg seem to share a more or less exclusively-held power to take control over another person's body, in Flagg's case so completely that the person does not even remember being possessed. The chief difference between them is that Mordred's skill does not seem quite as refined as Flagg's - even though, at the end of the Dark Tower series, Mordred is capable of possessing Flagg himself. Flagg and It both leave messages in bright red paint (or blood, if paint isn't readily available), typically with cheery overtones.

Such thematic links, however, can probably be attributed to an unspoken or under-appreciated tendency on King's part to keep certain similarities between his characters. In order for the theory to be represented, one must appreciate the existence of similar relations on different levels of the Tower, and how their fates are linked.

The "Twinner" Theory:
One of the more interesting (but less popular) theories concerning the Crimson King is that the King, Flagg, Mordred, IT, and many other villains are all iterations of the same archetypical evil, the ultimate representation of The Red — the "true" Crimson King, in other words.

In order for this to be reasoned out, certain thematic elements have to be established between each villain, or at least enough to make the point clear.

Twinners:
The idea of "Twinners" originates most clearly in King's original collaboration with Peter Straub, The Talisman. In the book, characters of significant importance have Twinners, different versions of themselves on different levels of the Tower - in this case, just the Territories. In general, the Territories seems to be a lot more selective concerning Twinners than the rest of the Dark Tower, in which entire worlds can be looked at as carbon copies of each other, except for whose face is on the dollar bill.

Basic rules governing "Twinners" are as follows:
Every person in the Territories has a twin somewhere on Earth (in most worlds of the Dark Tower, this is true of every person, so for the sake of argument we will assume every person has alternate versions of themselves in different worlds.) These twins, called Twinners, are by nature extremely similar - if not physically then psychologically or philosophically. For all intents and purposes, one twin could be incredibly different from the other (one might be an overweight businessman while the other one is a physically fit sorcerer), or the two could be so similar that they are more or less interchangeable. In general, the fates of Twinners are intertwined - if one dies, then the other tends to die in a similar fashion. The only cases where this isn't true are the deaths of Richard Sloat and Jack Sawyer's Twinners. In other worlds of the Dark Tower, this also tends to hold true - in the case where Jake Chambers was saved from the death shared by other versions of himself, he was nearly driven insane.

The Crimson King is arguably the single most important entity in the Dark Tower, and has on at least one occasion had a different iteration in at least one other level of the tower. Considering that, in every world shown in the Dark Tower except the Territories, there is at least one iteration of important people, it can be assumed that most worlds have some iteration of the Crimson King's archetype.

Twinners's identities have to be established through drawing some links in their basic fates. An incomplete list follows:

Flagg's apparent loss of power in Wizard and Glass, at least compared to his apparent power in The Stand, coincides more or less completely with the Kingfish's defeat in Insomnia.

It lives after Pennywise the Dancing Clown's apparent death in IT - this is signified in Dreamcatcher, in which a sign painted with Pennywise's trademark red paint claims "PENNYWISE LIVES." Pennywise apparently dies in IT, though it is not unimaginable that he survives; two possible explanations are that It and the version of the Crimson King that lives in Derry are the same entity, or that It died when the rest of the Crimson King's Twinners did not, contributing to their slow fall from power and the King's spiraling into madness.

Flagg's apparent death and rebirth at the end of The Stand (reminiscent enough of Jake Chambers' death in The Gunslinger that parallels can be drawn) would appear to coincide, in terms of time, with the first erratic behaviors of the Crimson King as chronicled in The Dark Tower VII.

Susannah Dean's becoming pregnant with Mordred Deschain coincides with Flagg giving up on having an heir of his own. This brings into question whether Flagg felt an empathic satisfaction of that need because the King felt it, but no other explanation seems to make much sense when compared to how vehemently Flagg was trying to have a son in The Stand.

As Mordred matures, he generally becomes more like It: he is able to control the minds or bodies of lesser people, his true form becomes less human and more spider-like, and in general his spider form becomes more unnatural and grotesque.

There are more examples, but they would take too long to list here. Taken in context, none of these seem to be any more than coincidence, but if they are grouped together and looked at in the light of King's tendency to link together everything significant in his books, they reveal something interesting - Flagg, the Crimson King, Mordred, It, and countless other King villains may have all been different iterations of the Red, embodiments of a single evil force. Essentially, this means that they are all the same person.

Such a claim would mean that several things are more readily made clear: Flagg's apparent fall from power, the Crimson King's spiraling into insanity, and Mordred's apparent quest to kill one or both of his fathers. All of this would come from the same set of events. Flagg's apparent death in The Stand, the events at the end of Black House, It's apparent death, the Kingfish's defeat in Insomnia, and even the death of Barlow in Salem's Lot could all contribute to the Crimson King's fall from power.

This also throws Mordred in an interesting light - it's arguable that, in killing Flagg and absorbing his power, Mordred was actually working towards re-establishing the King's dwindling power. It is probable that the final step would have been a meeting with the King himself - whether this would have resulted in Mordred killing the King, or the King killing Mordred, or some other exchange of power, is of course never made clear.

This theory is less accepted by the Dark Tower fandom at large, but is recorded and preserved for its uniqueness, if nothing else.