Sithis was mentioned in several books. But, Sithis as god of angsty death throes did make people say what the hell...even going as far as calling him Mephala even though books state they're two different entities.
If this was a hint to my previous statement, I feel like I need to explain myself. I did not say that void-Sithis and Mephala was the same being, I only meant that what the Cyrodiil Dark Brotherhood believe is a "god" called Sithis, is in fact Mephala in a clever disguise. Therefore, whenever someone "prays" to a shrine of "Sithis", they are really adressing Mephala.
We can't actualy think of sithis as a god of anything, the way i think of sithis is as a universe of pure energy, perhaps magicka, with a huge amount of power to both change and destroy, but not create. People think of sithis as more of a dark entity for thet particular reason, but in the monomyth, which i assume is acurate sithis is the biggest drivig force in the creation of nirn, or the entire universe of TES, as he started the stasis of Anuiel into action.
Sithis is nothing. THE nothing. Not existing, yet existing. try not to think of Sithis as a god or a person. Think of Sithis as an idea.
after doing my reseach on sithis, it has become very apperent that he is very far from nothing, i would like to put that idea to rest, as sithis is almost always described as chaos, and infinite change, and the only thing like that is pure energy, so sithis is nothing material yes, and he is absolutly nothing permanent, but he is just as much existant as light of fire, well as far as TES goes atleast lol