The Crimson King is a trans-dimensional demon/were-spider that acts as the main antagonist of the Stephen King novel, Insomnia, as well as the Dark Tower series. He is described as a demonic creature born of chaos who rules over Evil and many if not every demonic creatures in several universes of the Stephen King Mythos. He is the ultimate enemy of the Dark Tower's protagonist, Roland The Last Gunslinger.
In the Dark Tower series, the Crimson King enslaves many powerful psychics known as the Breakers to destroy the Dark Tower, the base of every existing universe, so that everything in existence would collapse with, being replaced by a chaotic darkness that he could rule and manipulate forever.
The Crimson King first appears as a menacing and dangerous Dark Lord, but he fittingly becomes more and more insane as the story progresses, ultimately murdering everything around himself and ultimately himself. After his demise, he becomes an undead abomination who attacks the protagonists and is ultimately defeated as he is erased from existence through a magical picture that represents him being created then erased.
According to supplementary works about the series, the Crimson King was born of a union between an elder demon of the primordial 'Prim' (the chaos/nothingness before the cosmos formed) and the difficult-to-place icon of Roland's world, King Arthur Eld. Arthur Eld was seemingly the first unified ruler of the world since the Great Old Ones had caused an apocalypse on many levels, a destruction so thorough, all was thought lost. Whether Eld was supposed to have any connection to the legendary/historical Arthur of our time and place is never spoken of. Celebrating his united crown, Eld and his court threw a celebration that was stated to be open to anyone. Using this opening, the Spider Queen demon and her followers assumed pleasing Human forms and reveled with the Human celebrants, with the Queen seducing Eld and producing the Crimson King. Since Roland is also a descendant of Eld, the two are distantly related.
The King supported the rise to power of pawns such as Randall Flagg and John Farson in his quest to drive All-World to its final ruin, and the collapse of the Tower. Through a particularly twisted and convoluted scheme, he caused Susannah Dean, apprentice gunslinger under Roland, to become impregnated with a child who would be both his own and Roland's, giving this child access to the Dark Tower the King could never have. The child, tellingly called Mordred, was all his fathers were, squared and cubed, but their flaws were also magnified in him, and he in essence self-destructed. This last setback seems to be what finally drove the King irredeemably insane. Even after his erasure, the King endured as a body-less wrath, reduced to a set of eyes, becoming his own sigil. Because Roland's journey was revealed to be a cyclical one, it is unknown whether he defeats the King the same way in each iteration. While a creature of pure nightmare, he is able to be defeated by mortals, and his downfall embodies Stephen King's idea that while true evil seems alluring and seductive from a distance, up close it is usually quite pathetic.
The Crimson King is portrayed as a intelligent creature bent on destroying the multiverse by knocking down the Dark Tower so he can remake it in his own image. Despite being intelligent, over the course of the book series he seems to grow more insane and his homicidal urges seem to happen more often. He tends to promise his allies power and prestige but also tends to demand that they serve him without any question. He shows a creature with no problem of having anyone including young children murdered as he is known to of ordered his servants to have children murdered in order to secure his plans go without fail. The Crimson King also seems to be a rather paranoid creature that fears a prophecy that he will meet his end to the point he eventually kills all those around him and later himself so he can become an undead abomination that retains not only his mind but powers. This is so no conventional means could kill him and he could continue his mad quest to remake the universe into a chaotic void to rule for eternity.