Darth Bane restructured the cult, so that there
could only be two - no more, no less - a master, and an
apprentice. Bane adopted cunning, subterfuge, and stealth as
the fundamental tenets of the Sith order. Bane took an
apprentice. When that apprentice succeeded him, that new
Sith Lord would take an apprentice.
However, the Sith quietly continued for centuries,
until the time of Darth Sidious. It was Sidious who 
was responsible for the revenge of the Sith against 
the Jedi. His measured and carefully engineered plot spanned decades.
Sidious was apprentice to Darth Plagueis, a wise Sith Lord whose 
knowledge of arcane and unnatural arts was reputed to extend to manipulating 
the very essence of life.