The Crimson King is the great mastermind in the rebellion against Gilead. He is the one John Farson takes orders from and he has tried to take Roland Deschain's mind away with Merlin's Grapefruit.
Although not much is known about him, Roland Deschain knows that this is the worst kind of evil, and will become his greatest advisory.
The Dark Tower VII
At the outset of the final installment of our saga, Roland’s ka-tet is scattered across several different wheres and whens. Susannah Dean (still in the clutches of the demon Mia) is in End-World’s Fedic Dogan: a chamber of horrors where magic and technology can be merged and where a monstrous half-human child can be brought forth into the world. Eddie Dean and Roland Deschain are in Maine, 1977, searching for the site of otherworldly walk-in activity, and a possible doorway back to Mid-World. Jake Chambers, Father Callahan, and the bumbler Oy are battling vampires and low men in New York’s Dixie Pig Restaurant, circa 1999, a place where long pig is definitely on the menu. As soon as our tet reunites, they must journey to the headquarters of Thunderclap’s Wolves in order to discover exactly why the Crimson King’s minions have been culling the brains of young children for twin-telepathy enzymes. The answer is more horrible than they realized, and bears directly upon Roland’s quest to reach the Dark Tower.