Thrawn was introduced in the Legends material, making his debut in the 1991 Timothy Zahn novel Heir to the Empire, being Commander of the Galactic Empires remaining forces after the defeat if Palpatine at the hands of his former Apprentice Anakin Skywalker (Darth Vader), where he faced off the Skywalker twins, Luke Skywalker and Leia Organa Solo.
