The Crimson King (first appeared in The Dark Tower VII: The Dark Tower)

The Crimson King is mentioned in the following books:

Insomnia (Sep 1994)

When Ed McGovern goes crazy, he says something (unintelligible, mostly) to Ralph about The Crimson King.

‘Finding out that she signed the petition was fate! Simple fate! Do you mean to tell me you don’t see that? Don’t insult my intelligence, Ralph! You may be getting on in years, but you’re far from stupid. The thing is, I go down to the supermarket to buy baby-food, how’s that for irony – and find out she’s signed on with the baby-killers! The Centurions! With the Crimson King himself! And do you know what? I . . . just . . . saw . . . red!’
- Part 1 / Chapter 3 / Section 1

Black House (Sep 2001)

“Eye of the King,” she repeats, and now it starts with the hands: kneading and twisting and squeezing and digging. “Abbalah! Foxes down foxholes! Abbalah-doon, the Crimson King! Rats in their ratholes! Abbalah Munshun! The King is in his Tower, eating bread and honey! The Breakers in the basement, making all the money!”
- Part 1 / Chapter 2

Duma Key (Jan 2008)

The antagonist in Duma Key, although female, appears in a red robe and is probably in league with (if not an incarnation of) the Crimson King.

I tossed back the top sheet on the pad and sketched again, this time using just the red pencil. Red, red, it was RED! The pencil raced, spilling out a human figure like blood from a cut. It was back-to, dressed in a red robe with a kind of scalloped collar. I colored the hair red, too, because it looked like blood and this person felt like blood. Like danger. Not for me but— “For Ilse,” I muttered. “Danger for Ilse. Is it the guy? The special-news guy?” There was something not right about the special-news guy, but I didn’t think that was what was creeping me out. For one thing, the figure in the red robe didn’t look like a guy. It was hard to tell for sure, but yes—I thought…female. So maybe not a robe at all. Maybe a dress? A long red dress?
- Part 3 / Chapter 6

The Dark Tower IV: Wizard and Glass (Nov 1997)

Graffiti on the Turnpike references the Crimson King.

Someone had spray-painted over both signs marking the ramp’s ascending curve. On the one reading ST. LOUIS 215, someone had slashed WATCH FOR THE WALKIN DUDE in black. On the one marked NEXT REST AREA 10 MI., ALL HAIL THE CRIMSON KING! had been written in fat red letters. That scarlet was still bright enough to scream even after an entire summer. Each had been decorated with a symbol.
- Part 1 / Chapter V: Turnpikin'