BBY/ABY is used to position items along the Star Wars Timeline. BBY refers to Before the Battle of Yavin. ABY refers to After the Battle of Yavin. The Battle of Yavin in Episode IV: A New Hope occured in 0 BBY/ABY.
Did you ever hear the tragedy of Darth Plagueis the Wise? It's a sprawling tale that establishes several important origin stories for characters fans know well. Readers learn how Plagueis seduced Palpatine to the dark side and how their alliance led to the Sith apprentice’s ultimate betrayal of his master. This is how darkness truly began to rise.
Darth Plagueis: Like all Sith Lords before him, he craves absolute power. But like no Sith Lord ever, he possesses the ultimate power—over life and death.
Darth Sidious: In secret he masters the power of the dark side, while publicly climbing to the highest government office.
One desires to rule supreme; the other dreams of living forever. Together, they will destroy the Jedi and rule the galaxy. Unless merciless Sith tradition becomes their undoing. . . .