Heir to the Empire was a position set to be claimed by Grand Admiral Mitth'raw'nuruodo ("Thrawn") around 9 ABY upon his return to the galaxy, where remnants of the fallen Galactic Empire awaited.
"Heir to the Empire" was the title set to be taken by Grand Admiral Mitth'raw'nuruodo ("Thrawn") around 9 ABY, the time in which he successfully returned from his exile on the extragalactic planet Peridea. Before his return to the galaxy proper, Thrawn's command was relegated to the forces at his immediate disposal. Those troops and starships were what remained of the 7th Fleet personnel who followed Thrawn into exile, forming an Imperial remnant force that persisted from the Imperial Era into the New Republic Era. Overall, however, that meant Thrawn's forces and resources were limited to what had survived the trials of exile or what could be found on the barren world.
His personal flagship, the Imperial-class Star Destroyer Chimaera, needed to be rebuilt but did see renewed use as the Grand Admiral's command ship. On Peridea, Thrawn also discovered allies in the form of the Great Mothers, three Nightsister witches descended from the fallen Witch Kingdom of the Dathmiri. The three witches helped Thrawn survive in exile, offered their dark magick for use in his designs, and allowed the Grand Admiral to make use of their personal fortress as a command station. The three witches respected Thrawn's authority and referred to him as their lord. Upon the Nightsister Morgan Elsbeth's discovery of Thrawn, her forces, including the mercenaries Baylan Skoll and Shin Hati, fell under Thrawn's command.
While on Peridea Thrawn's command was limited to the resources at his disposal, command over the galaxy's Imperial Remnants awaited Thrawn upon his return from exile. As "heir" to the Galactic Empire, former Jedi Ahsoka Tano believed Thrawn would assume control of what remained of the regime. Indeed, as per the Shadow Council's designs, Thrawn was set to command what remained of the Imperial Military and return it to glory. Despite his exile, Thrawn also represented a leader to the Shadow Council despite not being present at its meetings, technically falling into Gilad Pellaeon's delegation. All the same, a new authority other than Thrawn was set to emerge in the Shadow Council's plans via Project Necromancer, a cloning operation that had been of great interest to the late Galactic Emperor Sheev Palpatine.
The Chiss Mitth'raw'nuruodo—commonly known by his core name of "Thrawn"—was a tactical genius who served in the military ranks of the Galactic Empire, rising to the rank of Grand Admiral in the Imperial Navy. In that capacity, Thrawn launched a campaign against the growing Alliance to Restore the Republic until his defeat in 1 BBY at the hands of the young rebel and Padawan Ezra Bridger during the Liberation of Lothal. With his flagship Chimaera taken prisoner by a pod of Purrgil brought in by Bridger, Thrawn was dragged away into exile on the planet Peridea in the so-called far galaxy. While Thrawn and Bridger were away, the conflict between the rebellion and Empire escalated into open war and ended with the fall of the Empire with the battle of the planet Jakku in 5 ABY.
Lacking its central leadership after the deaths of Galactic Emperor Sheev Palpatine and Dark Lord of the Sith Darth Vader at the Battle of Endor in 4 ABY, with the subsequent public leader of the Empire—Grand Admiral Rae Sloane—escaping into the Unknown Regions after the defeat at Jakku and following the death of Counselor to the Empire Gallius Rax, the Empire formally surrendered on the order of Grand Vizier Mas Amedda via the Galactic Concordance. While Sloane worked on her plans to restore the Empire in the Unknown Regions, thereby continuing Palpatine's Contingency and unknowingly playing into his plans to return to the galaxy after cheating death, the Empire in the galaxy proper would be reduced to a series of remnants that, based on what the victorious New Republic saw, lacked any form of central leadership. In reality, a Shadow Council that united several remnants emerged but elected to hold back from any open shows of strength.
All the while, Thrawn had remained trapped on Peridea, where his forces dwindled in number but remained loyal. Of those forces, the resources at Thrawn's disposal included a legion of stormtroopers—who in time took on the name Night Troopers—who had managed to survive the exile. The stormtroopers would be marshalled under Captain Enoch, the head of Thrawn's personal guard. In terms of starships, Thrawn's forces that persisted through his exile included several LAAT/le patrol gunships and TIE/ln space superiority starfighters, while the Chimaera was left in a damaged state. Meanwhile, Bridger went off on his own path across Peridea. Eventually, Thrawn discovered the slumbering Great Mothers. Thrawn established an alliance with the witches on the promise that they could return him to the galaxy, with Thrawn promising to deliver them and their cargo to the Nightsister planet Dathomir in the main galaxy should they escape. To that end, he began the process of rebuilding the Chimaera. Guided to Thrawn's location by the Great Mothers, an old ally of Thrawn and former Nightsister, Morgan Elsbeth, led the effort to return Thrawn to the galaxy under the hope his return could bring about the resurgence of the Nightsisters. To reach the far galaxy and return home once they arrived, Elsbeth began construction of the Eye of Sion hyperspace transport ring.
Captain and Shadow Council member Gilad Pellaeon, himself a former subordinate of Thrawn, was aware of the effort to return Thrawn to the galaxy and promised the council it would soon come to pass; per the Shadow Council's designs, Thrawn's return would galvanize the Imperial Military back into a powerful force, thereby giving time for Project Necromancer to be completed. Indeed, Thrawn's return promised to galvanize the Imperial Remnants for a new war against the New Republic, with Thrawn set to claim the position of "heir to the Empire" as the regime's new leader. By circa 9 ABY, although Shadow Council member Gideon had heard no whispers that provided credence to the idea of Thrawn's return, rumors had begun to swirl of Thrawn's imminent return. Hearing those rumors, former Jedi Ahsoka Tano made an effort to prevent Thrawn's return. Reaching Peridea in the end, she, her apprentice Sabine Wren, and their droid companion Huyang failed to stop Thrawn, his personal remnant force, and the Great Mothers from escaping back to the galaxy aboard the Eye of Sion-docked Chimarea. Although Elsbeth was killed by Tano during the battle and Bridger managed to sneak back to the galaxy aboard the Chimaera, Tano, Wren and Huyang found themselves stranded on Peridea, but Tano promised Wren they were where they needed to be.
Successfully returning to the galaxy proper aboard the Chimarea and Eye of Sion, Thrawn, his forces, and the Great Mothers made their way to Dathomir with the Nightsister cargo.
The title of "heir to the Empire" was used by Ahsoka Tano to describe Thrawn in the Ahsoka episode "Part Four: Fallen Jedi." However, it would first be used publicly in the teaser trailer for Ahsoka, where dialogue was altered to combine Tano's lines from "Part One: Master and Apprentice" and "Part Four." Within the teaser, Disney+ subtitles for "Part Four," and episode guide for "Part Four," the h in "heir" is not capitalized. However, it was capitalized in the official trailer for Ahsoka released on July 11, 2023.
The name "heir to the Empire" has its roots in Timothy Zahn's Star Wars Legends novel Heir to the Empire, which marked the first appearance of Thrawn and kicked off the Star Wars Expanded Universe later rebranded as "Legends."