It's my understanding that per Legends canon, the Sith are/were a Force-capable race who were enslaved by powerful Dark Side users. These tyrants therafter referred to themselves as Sith Lords. Their order expanded and grew more and more powerful, until their defeat in the Sith War at the hands of the Jedi.
After that, the Rule of Two allowed their order to survive under the radar of the Jedi. But this question got me thinking: who decides who the Two are? If Darth Maul was stripped of his title and replaced by a new apprentice under Sidious, what exactly prevents him from declaring himself the new Master? Or if someone trained himself as a Sith and had no idea who (or where) the current Two are, could he start his own?
Disney or Legends answers accepted.