Mantle of the King...
The King never actually appears in the original Chambers stories. Although he is described by the narrator of 'The Yellow Sign' as having "opened his tattered mantle," this is not something he sees. Given that this takes place when the watchman breaks into the artist's studio, is it the King manifesting through his servant, or a delusion of the watchman assuming the form of his King? Again, we never really know, the experience being intensely personal to the narrator. The victim of 'The Court of the Dragon' only hears his voice, whispering to his soul. Given this, it's very hard to work out a description of what (or who) the King is, and how he appears.
Combing through the book highlights a few references...
"The ambition of Caesar and of Napolean pales before that which could not rest until it had seized the minds of men and controlled even their unborn thoughts," said Mr Wilde. "You are speaking of the King in Yellow," I groaned with a shudder. "He is a king whom emperors have served."
Dialogue between Castaigne and Wilde is frequently veiled in the trappings of their conspiracy, which may be, to varying degrees, delusional or may be completely independent of their madness. This is the first line of Chamber's book that tells us anything about the King, actually telling us little, but implying much. As a king whom emperors has served, presumably unknowingly, it is worth considering whether this is a mix-up of hierachy, or whether the title 'king' is less to do with rank than it is a sense of grandeur or power. Perhaps the role is purely to conform to an archetype.
In terms of kings and emperors, it might be argued that the former is much 'closer to the people' than the latter, and a Kingdom much more centralised (and controlled) than an Empire. Emperors are often seen as warriors, Kings more as noblemen (in very basic terms - Kings are born, Emperors struggle to be something greater).
And yet the King in Yellow 'could not rest until it had seized the minds of men and controlled even their unborn thoughts', which suggests the King might have the urges of conquest and control usually equated with an Emperor. It might be that the realm of the mind, of conscious and unconscious thought (or some aspect of it), is 'his' rightful Kingdom. It's also interesting to note that the phrase is set in the past tense. The King has already seized minds and controlled unborn thoughts. Obviously not all minds, so the relevance of the quote must lie elsewhere (unless truly the ramblings of a madman). Might it hint at some distant time when the King was first able to manifest some power in this world? Or might it relate to those who have fallen beneath 'his' rule in alien worlds, much like Carcosa?
Of course, all talk of Kingdoms and Empires might be misleading. If the King insteads represents a parody of ruling nobility, or a metaphor for power (and control) the borders of his 'Kingdom' are unmarked and may be ever changing.
It is a diadem fit for a King among kings, an Emperor among emperors. The King in Yellow might scorn it, but it shall be worn by his loyal servant.
Another allusion to Kings and Emperors, suggesting that the King in Yellow is just as much an Emperor as a King (note the use of capital initials), but also suggesting his distaste for the trappings of royalty, highlighting his more subversive role. Castaigne, the narrator, has no such objection. In many ways there is a parallel with the opinion some religions have of material possessions. In this way, Castaigne is only human and, by clinging to the trappings of power (rather than the power itself) fails to grasp the transcendent messages of his God. Perhaps this preoccupation on mortal concerns (in particular, the usurping of his cousin's throne) that leads to his downfal.
It's interesting to note that, aside from his own misguided understanding, Castaigne seems relatively unplagued by the King. Whilst most of the characters in the Chambers stories fear the play (potentially attracting the King with their fear), Castaigne's fear is tempered but his loyalty to the King. Does the King favour Castaine for his loyalty, fail to notice him at all for his lack of terror or just allow him free reign to exercise his own madness? Of course, it's always possible that the whole story takes place after Castaigne has been cursed by the play and the King, and his miserable fate in the story is an inevitable conclusion.
"The scalloped tatters of the King must hide Yhtill forever," he muttered, but I do not believe Vance heard him.
A descriptive piece that suggests that the King may well dress the part of the King, even if 'he' chooses not to keep them from becoming ragged. Or, again, the clothing is a parody of regal dress.
(As an aside, it is worth pointing out that the phrase ‘King of Rags and Tatters’ is a title heralding back to medieval times, where troupes of actors would perform what was known as ‘The Mysteries’. Although there were many characters, this particular character was called, or represented, ‘Vice’. He would be dressed as a mockery of a King, in ragged finery, acting much the same way as the traditional ‘Fool’…)
As for what Yhtill is, we never know. Whether it is the name of a place (a city?), a person (the King?) or something else entirely isn't clear, and this is the only time in Chamber's book the name is used.
It is also curious to note that the King's tatters 'must hide' Yhtill. Does this mean they must EVENTUALLY hide Yhtill to achieve the King's grand plans or that they must CONTINUE TO hide Yhtill to avoid incurring the King's wrath (or the threat of something else)? Or is it the less likely suggestion that these tatters must SURELY hide Yhtill just because no-one has ever found 'it', and have next to no chance of doing so. This idea, of course, reduces Wilde's words to casual observation rather than words of portent.