Rule of two - Drew Karpyshyn

In this essential Star Wars Legends novel, the second in the Darth Bane trilogy, the fearsome Sith lord takes on a deadly new apprentice.

Darth
Bane's twisted genius made him a natural leader among the Sith-until
his radical embrace of an all-but-forgotten wisdom drove him to destroy
his own order . . . and create it anew from the ashes. As the last
surviving Sith, Darth Bane promulgated a harsh new directive: the Rule
of Two.

Two there should be; no more, no less.
One to embody the power, the other to crave it.

Now
Darth Bane is ready to put his policy into action and thinks he has
found the key element that will make his triumph complete: a student to
train in the ways of the dark side. Though she is young, Zannah
possesses an instinctive link to the dark side that rivals his own. With
his guidance, she will become essential in his quest to destroy the
Jedi and dominate the galaxy.