The Mandalorian has finally brought Ahsoka Tano to live-action, and her epic introductory episode also reveals what she's been up to: searching for Grand Admiral Thrawn. While viewers who have only seen the Star Wars movies might not know who that is, fans of the animated TV shows know that Thrawn is very, very bad news, and an old enemy of Ahsoka's who might still be stirring up trouble.
Throughout the episode, Ahsoka is battling with Morgan Elsbeth, a woman who has significant ties and loyalties to the remnants of the Empire, over some piece of information that Elsbeth has and Ahsoka wants. Eventually, Ahsoka defeats Elsbeth in a fight and demands information about Elsbeth's "master," Grand Admiral Thrawn. The episode ends without confirming for us whether or not Elsbeth gives up any of the information Ahsoka wants, but the mere fact that Thrawn is still out there is not a good thing.
Thrawn is best known for being one of the biggest bads on Star Wars Rebels, the animated series that takes place during the Rebellion era, about five years before the original Star Wars trilogy picks up. His backstory is also covered extensively in a series of canonical novels. In brief, he's a member of the alien race Chiss, a ruthless Imperial officer who became one of the most powerful men in the fleet. During the era of the Rebellion, he came into direct conflict with the Rebel cell led by Hera Syndulla, which also included Ezra Bridger, a young Jedi, and Sabine Wren, a Mandalorian.
One of his biggest arcs, however, was his attempt to root out the spy known by the code name "Fulcrum" — who, at least at first, was Ahsoka herself. Ahsoka came up with the alias as a reference to a frequency that her friend and mentor, Anakin Skywalker, had used during the Clone Wars. Although Ahsoka was the first Fulcrum, the name came to be used as a standard codename for informants, including Alexsander Kallus, an Imperial agent who became a Rebel informant. Thrawn spent a significant amount of time trying to discover Fulcrum's identity, even after Ahsoka left the name behind when she was presumed dead. He was known for being particularly clever and particularly brutal in equal measure, responsible for several horrible attacks and battles throughout the series.
Thrawn was one of the key figures during the war, advising the Emperor and Darth Vader directly and being personally responsible for several Imperial technical upgrades and intelligence strategies. His military might, however, was no match for the plans of the rebels in the end. After he captured the young Jedi Ezra, he believed his path was clear to destroy the planet Lothal. Instead, he fell victim to Ezra's plan: summoning a bunch of purrgil, enormous tentacled creatures that wound up sucking the entire ship — with Thrawn and Ezra on it — into hyperspace, where no one knew where they were.
That's the last we see of Thrawn in the current continuity, but in the pre-Disney canon of novels (now referred to as the "Legends" continuity), he played a big role in the post-Empire era too, leading Imperial remnants to hassle and rebel against the New Republic. The Mandalorian certainly implies that at least some of that is still the case in this version of the universe, given that Ahsoka is still looking for him several years after the fall of the Empire. She doesn't reveal what she believes he's been up to, but it's definitely nothing good, and we're very curious to see if and when he reveals himself again — and for what evil purposes.