On my reread of NoK, the way that ICE refers to the Deadhouse and Shadow House got me thinking. He refers to them both as "the House" constantly. I don't think SE has ever referred to either of them as "the House." SE usually uses the term "Shadowkeep", not "Shadow House"; he refers to the Deadhouse as an Azath, and I don't think the word "Azath" is even in NoK.
This might be a little simplistic, but the descriptions of the Shadow Moon event are noticeably physical, not just magical.
Night of Knives, Tor TP, p.63 said:
Night of Knives, Tor TP, p.258 said:
Although I don't have quotes to back this up, Shadow is often described as wandering over the physical realm, like another layer of reality that slides over the world. What if the Shadow Moon was simply the convergence of the physical locations of Shadow House and the Deadhouse? An eclipse is a good metaphor; imagine Shadowkeep to be a small keyhole to the Shadow realm, one that allows power and control over it. We already know the Azath are portals to other realms, another keyhole. So the night of the Shadow Moon, those two keyholes align (as in, physically overlap and occupy the same space) and passage through the Deadhouse would allow access into Shadowkeep.
Shadow House generally cannot be accessed from the outside. When Kiska stumbles upon it (on her way to the Deadhouse, no less), she is entranced by its black glass and Edgewalker has to stop her from getting too close to it. (After Edgewalker spits her out of Shadow, she tries to escape a cultist and hits her head...she awakes at the wall at the back of the Deadhouse. From Shadow House to the Deadhouse without actually going anywhere.)
Night of Knives, Tor TP, p. 232 said:
'Really? That? But it's-'
'Alive. Quite so. And very dangerous.'
Azath houses are also described as being "alive." But it's dangerous to get close to from the outside, also like an Azath. Once inside, though, you pretty much have free reign. So how to get inside Shadowkeep? That would seem to be the only way to claim the Shadow realm and become its ruler.
Also, I have often seen it said that Oleg's entombment comment refers to being captured by the Azath, similar to this quote:
Night of Knives, Tor TP, p.215 said:
But:
Night of Knives, Tor TP, p.81 said:
'The price of failure. Eternal enslavement to Shadow House.'
Edgewalker comes out and says that entombment is enslavement to Shadow House, not the Azath (though it certainly must be both, and Oleg had to be referring to the Azath). Clearly Shadow House also traps those who try to assault it, just as an Azath does. Could Shadowkeep be a "shadow" of the Azath? That is, a physical reflection of an Azath in the real world? Physical objects often have counterparts in Shadow, as seen by Apsalar in tBH and even in NoK with the Stormriders. They approach physically from the south where Obo, Tay, and Agayla; but a huge glacier is also seen approaching Shadow House at the same time.
This seems to answer a number of questions. 1) Why is the Deadhouse the only Azath that Kel and Dancer could use to gain access to Shadow? After so many years of traveling the Azath and residing in the Deadhouse, they must know of multiple physical Azath locations. What makes the Deadhouse special? 2) Why did they have to access it on the night of the Shadow Moon? They had entered the Deadhouse many times before, probably hundreds. What was different about that night? I feel both of those questions are relevant and are answered by the eclipse/keyhole theory.
I think that the transubstantiation bit is less relevant to ascension than entrance into the Shadowkeep; taking control of an entire warren would be grounds for ascension, I think. My guess is that being a "shade" like Oleg made Kel and Dancer less noticeable to the Azath; with a huge number of physical, living beings fighting right on its doorstep, two dead or half-dead shades are lower on its radar than say, Temper. We see Kellanved actually crawling through the Azath's yard on his hands and knees; normally just setting foot into the yard is asking to be enslaved. So, in typical fashion, the showdown with Surly both finishes their role in the Empire and makes them less noticeable to the Azath. When Edgewalker comes into Shadow House (just like a Guardian would be allowed in the Azath after being enslaved), he finds Kellanved lying on the ground, which to me implies recent entry and exhaustion. Sure, he plays a little trick on Edgewalker with the fake body, but why lie down at all if you didn't have to? He even asks Cotillion to help him up. This means to me that they entered Shadowkeep directly; they did not enter Shadow first and then Shadow House, but appeared inside it. This was the key to claiming the throne and ascending. It also connects Kel and Dancer's long study of the Azath with their rule of Shadow - it wasn't just the path to Shadow, but the actual nature of the Shadowkeep and the throne inside it.
We have seen multiple links between Azath and Shadow - Tremorlor anchoring a fragment of Kurald Emurlahn used by the Whirlwind Goddess, Icarium's assault on the Jhag Odhan Azath somehow further shattering the warren - but nothing this direct, I think.
Alright, crazy theory time over.