do you go to the Void to serve Sithis or not, or is there more to it then that?

Unless the assassin traps you in a black soul gem, I'd say you go into the dreamsleeve process of rebeing. I'd say the Night Mother was a 'rebel' from the Forester's Guild who had Mephala's favor, so in death she was able to guide fellow rebels along her whims. The Daggerfall mention of the Morag Tong cites their worShip of Sithis as unambiguously worshipping the idea of Murder, or one of Mephala's spheres of goverence. Murder, in general, in Secret, the Act not the Plot.

Unless the assassin traps you in a black soul gem, I'd say you go into the dreamsleeve process of rebeing. I'd say the Night Mother was a 'rebel' from the Forester's Guild who had Mephala's favor, so in death she was able to guide fellow rebels along her whims. The Daggerfall mention of the Morag Tong cites their worShip of Sithis as unambiguously worshipping the idea of Murder, or one of Mephala's spheres of goverence. Murder, in general, in Secret, the Act not the Plot.