Dark force rising - Timothy Zahn

In this essential Star Wars Legends novel, the sequel to Heir to the Empire, Grand Admiral Thrawn's sinister plan threatens to spell doom for the fledgling New Republic and its most iconic heroes.

The
dying Empire's most cunning and ruthless warlord, Grand Admiral Thrawn,
has taken command of the remnants of the Imperial Fleet and launched a
massive campaign aimed at the New Republic's destruction. Meanwhile, Han
Solo and Lando Calrissian race against time to find proof of treason
inside the highest Republic Council-only to discover instead a ghostly
fleet of warships that could bring doom to their friends and victory to
their enemies.

Yet most dangerous of all is a new Dark Jedi,
risen from the ashes of a shrouded past, consumed by bitterness, and
scheming to corrupt Luke Skywalker to the dark side.