The Crimson King
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.
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 manipuliate 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 represent him.