Sithis is one face worn by many different forces and beings.
It refers to the Padomaic essence, from which some say Lorkhan was begot.
It refers to the Dark (a word which the frail mortal mind uses to approximate a condition beyond its understanding) antithesis to Anuiel, those silences that translate the white noise of Light into music.
It refers to the Psijic impetus of chaos, a linguistic corruption of that Order's deliberately unpronounceable PSJJJJJ.
It refers to a void-god worshiped by the Dark Brotherhood, and seems to have some power over the souls of its servants.
It refers to an ancient spirit by which the twelve worlds of creation were connected, and is recognized by the Hist of Argonia to this day.
Some will tell you that Sithis doesn't exist, and they are correct. But Sithis does, and so does Sithis. Sithis is really just a misnomer, but Sithis is the true name of Sithis, and Sithis exists independent of them all.
I say this with my Sithis Face.