I read on another thread that Nocturnal is said to be 'an echo of the Void', and the Void is the Sithis-aligned domain. But the link is only tangential IMHO. Which in TES can actually be a thing of substance, of course - but even so, a thin one here I think.

My personal theory is that Mephala is both the Nightmother and Sithis. Mephala has been referred to as both male and female. Depending on who's talking and what book it's in. I feel that they are aspects of the same entity. And that a mortal 'Nightmother' is more in reference to a station within the DB. In Morrowind, you end up going on a scouting trip for the Sanguine items. Some of these items are found on 'the Nightmother'. I think it's something like being called 'Listener'. Mephala loves to stir things up. So why not continue having the Morag Tong and the DB poke at one another? Most of the Sanguine items are found on DB members. It's all a good giggle for Mephala. Granted they're all her/his loyal children. How many parents have a bit of a joke on their kids?

So, I stick by my theory that it's all an aspect of Mephala.

About the "Thieves Guild in Cyrodil is a Myth" retcon in Oblivion, the game hints that it is Nocturnal's curse on the Grey Fox that made others think of it as a myth. As it explains, the Grey Fox's identity was eliminated from history, so that everything he did and made could not be attributed to anything. The thieve's guild of Cyrodil, as an institution of the Grey Fox, became the Grey Fox itself, with Corvis Umbranox(sp?) as the head, and as the figurative head, he also wore the cowl of Nocturnal, so that made the entire Thieves guild of Cyrodil just as Mythical as the person was. They could be open in recruiting, because those that did not become part of the Grey Fox forgot about it, with fate selecting the parts of the Thieves Guild.

"The beggers are the eyes and Ears of the Grey Fox." I believe this to be literal as well as figurative. They are just as forgettable and overlooked, as stated in the book "Beggar Prince." Just like the Grey Fox.

The thieves guild members are like the hands of the Grey Fox, as evidenced in the various thieves guild quests where the PC takes on the role of the Grey Fox (Breaking the siege on the waterfront, reassigning Lex, etc). Evidence of the members being part of the Grey Fox is reflected in the Black Horse Courier's recounting of the events.

The Thieves Guild of Morrowind is not the Grey Fox, so it is not a myth, its just insignificant.

I don't consider it a "retcon" or any kind. The thieves guilds we've seen in various provinces appear to be unconnected to each other, for the most part. The thieves guild of the Illiac Bay didn't seem to answer to anyone from Cyrodil, and was much less "public" than that of Vvardenfell. As for Gentleman Jim Stacy, he didn't seem to be answering to the mainland, either. It might be strange that there was such controversy as to whether or not the Thieves Guild of Cyrodil even existed, but that was a peculiar aspect of their own leader, and not something that extends either to the Thieves Guild of the Illiac Bay, or to what may now be called the Bal Molagmar (IIRC).

I suppose that makes sense, but the whole "thieves guild is a myth" thing is still kind of lame.

It's not a retcon, as it doesn't contradict anything. It just looks stupid when compared beside it. And independent of it, really.