1. Well, Lord Revan went to the unknown regions to fight te True Sith. Than the Exile went after him. Now the true sith are just a Force-sensitive species that interbred with the Force Users that went to the Dark Side after the Great Schism. A few examples of those sith are: Naga Sadow, Marka Ragnos, Ludo Kresh. But they are all dead.
While i was reading Clone Wars(Light and Dark) there was a scene where Darth Tyrannus gave Quinlan Vos the crystal of Darth Andeddu, who looked like a true Sith.
Now i red on Wookiepedia that they have plans for him. Maybe they will use him a leader of the True Sith. http://starwars.wikia.com/wiki/Darth_Andeddu
2. Although the likehood of the things that i'm going to share is very small, i just want you to read it: In the new comics, Knights of the Old Republic the protagonis is Zayne Carrick. Zayne was a Padawan who was not very talented and always came to late. On the night where he was supposed to be a Jedi Knight he witnessed how his master and the other Jedi Masters of the council kils his fellow Padawans, shocked by this he runs away. While he runs away his masters tells the authorities it was Zayne who killed the Padawans and that they'll deal with him. After a little research he finds out, that his masters thinks he is going to be a Dark Lord of the Sith, a Destroyer of Worlds. Could this referring to Darth Nihilus?
http://starwars.wikia.com/wiki/Zayne
Or Darth Nihilus could be his friend, Squint
http://starwars.wikia.com/wiki/Squint
So maybe a game about this?
Greets, Darth Nihilus