In the time after the original “Star Wars” trilogy concluded and before George Lucas made the prequels, there was a serious gap in storytelling within the universe. Author Timothy Zahn helped to fill that void in a big way with his “Heir to the Empire” trilogy of novels, with the first installment being published in 1991. These books served as a sequel to the events of the original trilogy, with Grand Admiral Thrawn emerging as a new threat in the wake of Palpatine’s death. Luke, Han, Leia, and the rest of the former Rebellion leaders have to band together to stop Thrawn from returning the galaxy to Imperial rule.
Thrawn is wildly different from Darth Vader. For example, he is not a Force-user. Rather, it is his incredible mind that serves as his greatest weapon. Thrawn is a military genius and is known for using artifacts such as artwork to learn about his enemy. He is a member of the alien race known as the Chiss, with blue skin and red eyes, which is notable since you don’t typically see many aliens working for the Empire, let alone as part of the higher command. (The Empire is a little racist that way.) Yet, Thrawn’s brilliance could not be denied and he ascended the ranks anyhow. After the destruction of the second Death Star, he took it upon himself to reorganize what was left of the Empire in his own way.
The “Heir to the Empire” books are no longer canon as Disney jettisoned the former Expanded Universe, designating all of these tales as Legends. Basically, anything published before 2014 no longer counts as canon. But Dave Filoni brought Thrawn back in “Star Wars Rebels” season 3, making him officially part of the new continuity. In the years since, Zahn has returned to write two new trilogies of “Thrawn” novels as well, all of which are canon.