The Crimson King is a powerful immortal being whose characteristics mirror both the physical realm as well as the nature of the Prim. He is believed to be of the same species as the monster, IT, some even going so far as to believe IT is his son, but this is unofficial.
The King is the controlling power behind such villains as Atropos, Mr. Munshun, Dandelo, and Randall Flagg. He also controls many minions such as vampires, the taheen, the can-toi and a great number of humans via John Farson. He has a number of godlike powers, such as telepathy, shapeshifting, weather control, and mental suggestion, as well as the ability to travel and exist on different levels of the Dark Tower and throughout the multiverse.
The Crimson King was the bastard offspring of Arthur Eld, a legendary gunslinger and a demonic entity, the Crimson Queen. At some point, the Crimson King took the hierarchy of the Red, and had the plot to topple the Dark Tower, and destroy the reality it holds, so he can rule in the primordial chaos.
He is first properly introduced in Insomnia. He learns of Patrick Danville, a boy who is destined to have a part in the Crimson King's defeat. He and Atropos come with a plot to have Patrick killed in Derry. They manipulate Ed Deepneau to crash an explosive-rigged light plane into the Derry civic center, where a heavily attended women's rights rally is being held, with Patrick present. When Ralph Roberts confronts the Crimson King, he manages to force the light plane to crash, killing Deepneau, and saving Patrick's life, putting him on track for the Crimson King's defeat.
In Black House, he has a serial killer and willing host of Mr. Munshun, Charles Burnside, kill and cannibalize children. The children had the potential to serve as Breakers, humans who have psychic powers. Burnside is killed by Tyler Marshall and Munshun is killed by Jack Sawyer.
Breakers would be taken to a facility called Algul Siento, in Thunderclap, where there is a room that amplifies their powers into a corrosive and damaging force to the Dark Tower. The facility is sacked by Roland Deschain, his ka-tet, and rebel Breakers.
His sanity soon begins to deteriorate, and as it begins to worsen, he kills his minions and even himself. As a result, he becomes undead and immune to Roland's bullets. He finally reaches the Tower but becomes trapped on one of its balconies.
He finally confronts Roland and Patrick in the climax of the last book in the Dark Tower series. He has become powerless but has grenade-like weapons which he throws at them. As predicted, Roland and Patrick bring about the Crimson King's downfall. Patrick captures the Crimson King's image and with his supernatural artistic abilities, a mixture of Roland's blood and rose petals, Patrick erases the Crimson King from existence, possibly sending him todash between worlds.
Since the Dark Tower series is in a time loop, the Crimson King's status is unknown.