So, I was reading some dark brotherhood related books in Skyrim a while back, one was a commentary of a writer who was allowed to learn about the night mother's way of life, leaving the black and white rocks in her victim's eye cavities etc, and I'm not certain if it were the same book or a different one that went into a story that the thieves guild (if it were called that) used to do assassinations, which the "night mother" was a part of at this time, but for some reason the guild split up and the night mother went on to form a guild purely for assassinating and gave up thievery.
Correct my memory if I'm a little off, and please, elaborate on this story as much as you know, It interests me a lot, mainly because I've hope that with so little of the dark brotherhood left, and the thieves guild not doing that well any more, maybe the designers could take the opportunity to re-merge the factions in an expansion pack.

The book you're referring to is called 'Sacred Witness', and it offers just one of numerous intriguing theories for the infamous debate on who exactly the Night Mother is. I don't know if you've read them, but seeing as you're interested, 'Fire and Darkness' and 'The Night Mother's Truth' , would be interesting reads for you as well. Together with the Morag Tong questline in MW, and the DB questlines in OB and Skyrim they make up quite the conflicting account of who the Night Mother is.

Anyways, I don't think the DB and TG would ever merge as their interests would more than likely conflict. However, the Dark Brotherhood is on speaking terms with them in Skyrim, or at the very least is available to be contacted when necessary, though this is probably more out of necessity seeing as they now have to scrounge for any contract they can get. The DB is hardly the seemingly ominpotent force it once was by the time of Skyrim.

That's probably just one of the like half dozen different origin stories for the Night Mother and her children.

Perhaps she deliberately wants to cover up who she truly is by concocting several conflicting accounts.

Much like web-spinning.

I have come to the conclusion that the night mother and nocturnal are the same person/deity
split personality

TES background lore borrows a page from the Joker.

"If you have to have a past, make it multiple choice."