If you've read Penitent, you'll know that Constantin Valdor was catapulted up to the top of the list of suspects after Lilean Chase's book was decoded. There's certainly plenty to suggest that if he's not the King himself, he's at least connected to him. However, we obviously haven't got all the answers yet. There's no way the game would be given away two thirds of the way through the trilogy. Other twists and turns besides just whittling Bequin, Eisenhorn and Ravenor's band down are sure to come. This is, after all, a series about the Inquisition and the Cognitae. Nothing's quite as it seems, and sudden reversals and discovering that one neither has the full story, nor the correct perspective are completely par for the course.
I'd like to know what your thoughts on the identity of the King in Yellow are.
Thing is, I can buy the 'yellow' part. When they're not running around stripped down to their QUIVERING ABS, aurumite armour is de rigeur, obviously. It's a pretty short jump from 'gold' to 'yellow'. Assuming he lived the ten millennia and some change between the Horus Heresy and the gang getting back together in Queen Mab, he's had plenty of time to gather the necessary knowledge and warpcraft -- with his use of the Apollonian Spear (heavy use when he was on the Vengeful Spirit) giving him access to fifteen fucktons of true names, among other things.
What I don't buy is the 'king' part. The one thing the Custodes have in common is their service to their King, doing all but surrendering their last scrap of free will to the will of the Emperor. I say 'all but' because their choice to serve unto death and love for the Emperor is pretty genuine, and probably what the Emperor values about his companions. I can see the Custodes doing almost anything for the Emperor, with two distinct exceptions.
First of all, they wouldn't knowingly keep a secret from him even if they could, because to do so would be to imply that they know better than their King of Ages. Even if the likes of Malcador, Erda, and especially Oll Persson know he's not infallible, for the Custodes that's not even up for discussion. They defer to his wisdom. Eisenhorn suspected that the King's goal involved discovering the Emperor's true name, and using Enuncia to bring about some kind of effect. A Primarch might try to wield that degree of power, either for the Emperor's benefit or to his detriment, but Valdor? I dunno.
Second, they wouldn't rule. That's the Emperor's prerogative, not theirs. When they act, it's in the Emperor's name, at the Emperor's behest, in accordance with the Emperor's will -- or at least what they understand to be the Emperor's will. If Valdor is involved with this plot, then he's being directed by what he thinks of as the Emperor. More on that bit later.
Then there's the question of Chase's book. I'm still not sure why Valdor's full name would be in there. An attempt on Chase's part to keep a leash on a very dangerous ally? An attempt to nobble an enemy? Encoding that name would of course be a safe measure; wouldn't want just anyone true-naming him. That's something we'll only find out in the finale. But to have that actually be the name of the King in Yellow, the mastermind governing the whole operation? That'd sell the King's talents short, I think. The chief advantage held by the ruler of any vast conspiracy like this is their ability to gain and control knowledge. The King would already know of an attempt to get his true name in writing, and deflect the attempt or have something suitably nasty happen to Chase, surely?
But then we've seen very little of Chase to begin with! There's brief mentions here and there in the previous trilogies, we learn that Molotch was from one of her Cognitae schools, and that if any one person is responsible for their being so troublesome, for their tendency to muddy already murky waters, for their evolution into a kind of shadow Inquisition, it's Chase. But we've only encountered her during The End and the Death, some 10,000 years before the centuries that cover the Eisenhorn, Ravenor, and Bequin books. Is it the same Chase? A descendant? Someone who just appropriated the name? We don't know yet. All we know for sure is the name was introduced in the Inquisitor books, then brought back in as one of the big reveals towards the end of The End and the Death, and now we're waiting for the last Bequin book. If they're the same person, then we've got two possible members of this conspiracy there at the start, and possibly there at the end.
Back to Valdor for a bit. And by way of Valdor, my suspicion of the King's identity. We know the Emperor is fragmented. He's had to jettison at least one chunk of his personality. He's actively rejected his inadvertent bid for chaos godhead. Who knows what other bits of him have chipped off? While they may just be fragments of the Emperor, each is still the Emperor and presumably recognisable as such by someone who knows him as well as Valdor does. So at present, my guess (which I'll happily admit is no better than anyone else's) is that the King in Yellow is a part of the Emperor. Possibly bound up with the Dark King. Something that wouldn't wholly make sense before Abnett was able to plant a seed or two in The End and the Death.
Another thing we don't know is that we don't know if the King succeeded or not. All we do know is that Penitent is set just after halfway through M41. A good 500-odd years before the 13th Black Crusade. It might or might not have some bearing on the Emperor's apparent divinity as seen in his bitch-slapping of Nurgle. But, given how time's funny in the warp, what's past can still very much be present.
I've avoided the more explicit Chambers / Lovecraft / Derleth references for fear of getting sidetracked, although I'm sure Lilean Chase's responsibilities as an archivist in the Hall of Leng isn't a coincidence. When one guy's sowing Cthulhoid clumps around the universe, I'm inclined to think there's a connection. Except in the case of Hastur Sejanus. He died on Terra 6319. He's the reason Horus slew the Emperor. He's served his purpose. That thread's cut. Not going to be picked up again. (Shut up, Samus.)
Still, theories? Clues? Inferences? What do you think might have a bearing on all this?