Kind of the opposite for the DB in this case, because Arena originally described them as a cult worshiping Sithis, which they later went back to being in Oblivion.

I dont even remember them being in Arena so i cant really say much in that regard

It was in a blurb in the manual.

I will be honest that makes them sound much more brutal and less subtle than they are now but at least the foundation was well laid back then

Something gave them Shadowmere, something speaks through the Night Mother's corpse (and can unlock/create weapon enchantments - as seen with Oblivion's Blade of Woe) and Lucien said "the Void" (aka that something) doesn't want you to kill Cicero. It makes more sense for that something to be Mephala in disguise than a primordial force with no interest in the world at all.

I always thought Mephala was the Nightmother or playing off as her and using that corpse as a conduit. It makes sense to me if she were since she likes manipulating people like the Listener. Didn't she make the Morag Tong?

Nah, Night Mother is Vivec trolling around. Yea, Mephala gave the knowledge of espionage, cunning and plots to the Dunmer who worshiped her, they built a cult and order around what he/she taught and used it to help stabilize the houses and noble families so as not to start a house war.