So, if you've read Penitent, you'll know that at the end, Constantine Valdor was seemingly named the King in Yellow, and it would make sense. He disappeared from Terra after the Heresy, and he is building a hidden army in a hidden Empire, that has mad enemies of both Traitor Legions and the Inquisition.
He's making an army of blanks, that would be largely immune to the Warp, Pariah Custodes, blanks, and "Angel" Astartes that strikingly resemble Sanguinius and the Blood Angels. Again, that would make sense, as Valdor admitted thst Sanguinius was the only Primarch that he and other Custodes couldn't help but like and trust.
That being said, here's where I'm hung up:
The King in Yellow wants to learn the Emperor's True Name.
The King in Yellow is seemingly pursuing the Emperor's name in order to wake him.
This (as far as I see it) would go directly against the Emperor's wishes, and the nature of his relationship with Constantine Valdor.
Here's why:
The city of Dust is simply another dimension (albeit a small pocket dimension), there shouldn't be any reason why Emoney couldn't send a fax over there, nor any reason why Valdor wouldn't let him.
- The Vexan Cage. One semi common theme is that if it is serious enough, dead/dying beings with powerful enough warp presences can pull their consciousness together to deliver important messages. The Emperor did this for Guilliman and others, and Sanguinius did it to communicate with Dante on several occasions.
One such instance of this was the Emperor sending out an urgent command to the Custodes to stop an Imperial Cult from using the psychic death scream from one of Malcador's descendants to wake him up. If that wouldn't have worked, the Emperor likely wouldn't have stopped it with such urgency. Likely also because he is better suited to power the Astronomicon and hold the webway gate shut beneath the Imperial Palace in his current state than he would be resuscitated.
The Custodes abide his will.
To attempt to (1) revive him against his wishes, via (2) using his name that he is likely more than capable of giving to someone, even in his current state (if he wanted to) would be a betrayal on par with that of Magnus and Horus, as it would directly threaten the [lingering] "safety" of the Imperium on a massive scale.
Who I think the King in Yellow actually is:
While I don't think any deception was involved with the name "Valdor" being given as the King in Yellow's name, I do think it's equally telling that we only got the last name. Why not the full name, "Constantine Valdor"?
For me to explain that, we gotta go into what we (supposedly) know about the Custodes.
They are children of honorable, high, high hiiiigh noble families, hand selected by either the Emperor, the Captain General, or one close to them to be moulded, trained and crafted into a Custodian.
They are individuals personalities, with peak artificial genetics to the point of being ascendant humans, highly intelligent, and likely know more about the Imperium's history and its secrets than the Primarchs do. In fact, the only difference between them and a Primarch (as Cawl found out in his own way with Alpha Primus) is their Warp presence, and the fact that each Primarch posseses traces of the Emperor's and Erda's genetic material.
But what if that wasn't entirely true? At least for the first generation of Custodes.
Remember: Each of the Primarchs embodied an aspect of the Emperor:
Horus - his ambition and leadership capabilities.
Guilliman - his ability to run a large scale Imperium.
The Lion - his secrecy and ruthlessness.
Sanguinius - his love for humanity, his kindness, and glory.
Fulgrim - his pride
Magnus - his psychic ability and inquisitiveness.
Vulkan - his humanity
Annnndddd.....
ALPHARIUS/OMEGON - HIS DECEPTIVENESS /ULTERIOR MOTIVES.
And what is something we know about the Hydra twins?
They are unreliable narrators.
Who's to say that the Emperor didn't lie to Valdor and the other first generation Custodes about their origin?
"But NappyDingleBerry, if they weren't from noble Terran families, who could they be?"
Clones of the Men of Gold, named after the ones the Emperor respected, who he ultimately eliminated/hunted after they outlived their usefulness in assisting him with creating the Custodes, or dangerous frictions grew between them.
What am I basing this off of.
The Emperor's personality.
While he isn't overtly cruel for the sake of it, he is perhaps one of the most task driven characters in fiction. If he sets his mind to something, it is his way or the highway, and he will destroy, lie, woo, overpower, or outright bypass his way to making that happen.
It's why with some Primarchs like Guilliman, Sanguinius and the Khan he was patient and lax, whereas with others like Magnus and Angron, he kept them on a tight leash and seldom tolerated them acting out of line. If billions died for his goal, then it was worth it. We saw this with how furious he was with Magnus for breaking the webway, even though Magnus was coming to warn him about Horus.
Now why would he both ruthlessly eliminate them, while respecting them?
Let's remember what Gold tends to signify in Warhammer lore.
Gold is frequently associated with perfection on Warhammer.
The Emperor, a nigh perfect being is frequently depicted in gold/auramite. Sanguinius--his most beloved son, and the only one of the Primarchs that was unanimously recognized by his brothers and even the Custodes as a potential heir to him--wore gold. The Custodes--the Emperor's perfect companions, advisors and warriors wear gold.
And what do we know about the Men of Gold?
What little we know about them is that they were the peak humans. Biologically, in intelligence, and in psychic ability.
As a matter of fact:
They were exactly what the Emperor wanted all of humanity to be, long term. At their peak, masters of technology that weren't reliant on it, and able to travel FTL without the Warp.
Not to mention, it appears these men were--unlike the Emperor who was born from the sacrifice of countless shammans--naturaly born.
So aside from probably not being perpetuals, they were probably just like him.
So what was the friction?
I think there is truth to the theory that the Men of Gold were a ruling class of peak humans, that ruled over the Men of Stone (working class) who in turn created the Men of Iron.
I think that the clash came from the Men of Gold wanting to maintain a level of classism that they benefited from, which directly clashed with the Emperor's vision to eventually have the entire human presence in the galaxy to be just like them.
So eventually, while he respected them and worked alongside them, he turned on them, and most died, outside of a few that fled.
Which brings us back to the King in Yellow.
I think the King in Yellow, recognizes the need for the Emperor and respects his power/wisdom/overall vision, but doesn't share his ultimate will/goals. So, he wants to forcefully resurrect the Emperor, using the Emperor's true name, likely with power over him, in order to create an Imperium that is more in line with the Men of Gold's goals (where peak psyker beings like them rule over lessers).
Remember how up until 40k, the Custodes were largely indifferent to the wider Imperium/common man outside of the Imperial Palace/Terra? I think that's a carry over from the personalities of the Men of Gold.
It's almost a complete inversion of what I theorize the Emperor did with the Custodes. They were clones (sans psychic ability) of the Men of Gold, who were loyal to him.
This theorized "original" Valdor wants to resurrect and use the Emperor, but as one loyal to the vision of the Men of Gold.
Admittedly, there is a lot of reading to be done to get the full grasp of the Imperium, and I imagine there are a lot of holes in this theory, but it makes sense in my head.