Grand Admiral Thrawn
In the "Thrawn" trilogy book series, the New Republic is almost brought to its knees by Grand Admiral Thrawn, the new leader of the remnants of the Empire (which are known as Thrawn's Empire or Empire of the Hand) and a military genius. By the time of the third book in the series, Thrawn has nearly defeated the New Republic, but they claim victory in a last-ditch effort, and Thrawn is killed by his own bodyguard, shattering the Empire's unity.