Two there should be; no more, no less. One to embody the power, the other to crave it. The Rule of Two.
The Darth Bane trilogy is part of the expanded Star Wars Universe and likely a nice read only for the fans. Darth Bane is the one that installed the rule that only two Sith Lords can exist, otherwise, they will always fight each other, instead of fighting the Jedi.
The trilogy is divided into three parts: Darth Bane: Path of Destruction, Darth Bane: Rule of Two and Darth Bane: Dynasty of Evil.
The books present the story of an evil lord, which starts as a miner on a desolate colony. After an incident, he has to run and joins the Sith army, where he is noticed as Force-sensitive. Trained at the Sith Academy, he quickly climbs ranks and becomes powerful and convinced that only letting two Siths to live could ensure the victory over Jedi.
He manages to trick the other Siths to fall in his trap and kills them all, taking a little girl as apprentice, Darth Zannah. Numerous other adventures follow the two, which lead to their final duel, where Bane is defeated.
The book is imaginative and a real pleasure for the Star Wars fans, while a bit dull for others. The story expands the Star Wars universe and fills some gaps in the original story. The writing has many gems of wisdom, from an evil perspective. The writing style of Karpyshyn is engaging, but not fast-paced or bursting with action. The action scenes are controlled, violent and short.
The trilogy is ok, but not recommended unless for die hard fans.