In last week's episode of Ahsoka, Grand Admiral Thrawn learned that Ahsoka Tano was trained by Anakin Skywalker. The news appears to take the formidable Imperial by surprise and much has been said this past week about the otherwise cool, calm, and collected villain's reaction to that particular revelation.
Why did the mention of Anakin cause Thrawn to stop and take stock before figuring out how best to defeat Ahsoka? Well, it all goes back to the events of Timothy Zahn's Thrawn: Alliances novel.
The second chapter in a trilogy which includes Thrawn and Thrawn: Treason, the in-canon story reveals that he and Anakin met on Batuu during The Clone Wars. Thrawn was still allied with The Chiss Ascendancy at the time and was there to stop a plot by the Grysk which would affect his people; Anakin, meanwhile, had embarked on a mission to save Padme Amidala.
They fight side-by-side and win the day, earning each other's respect in the process (in fact, Anakin mentioning Thrawn to Palpatine is why the then-Chancellor ultimately recruits the alien to his Empire). Decades later, the Emperor - aware of their history - sends both the Grand Admiral and Darth Vader back to Batuu to investigate a disturbance in the Force.
Thrawn deduces that Anakin and Vader are one and the same, but the Sith Lord makes it clear to his ally that the man he once knew is now dead. They agree never to speak of the Jedi again, though not before Thrawn makes it clear he will forever honour Anakin's legacy.
Zahn largely portrayed the character as an anti-hero in his books, but that's not the direction Dave Filoni seems to be heading in. The sheer amount of time Thrawn spent as part of the Empire was likely enough to warp his mind, while there are those who believe his time on Peridea - and an alliance with The Great Mothers - has driven him insane.
So, yes, Thrawn knows what Anakin and Vader are capable of, hence why Ahsoka being trained by the former causes him to reconsider his options!
Set after the fall of the Empire, Ahsoka follows the former Jedi Knight Ahsoka Tano as she investigates an emerging threat to a vulnerable galaxy.
In addition to Rosario Dawson in the title role, the series stars Natasha Liu Bordizzo as Sabine Wren, Mary Elizabeth Winstead as Hera Syndulla, the late Ray Stevenson as Baylan Skoll, Ivanna Sakhno as Shin Hati, Diana Lee Inosanto as Morgan Elsbeth, David Tennant as Huyang, Lars Mikkelsen as Grand Admiral Thrawn, and Eman Esfandi appearing as Ezra Bridger.
Ahsoka was written by Dave Filoni, who executive produces alongside Jon Favreau, Kathleen Kennedy, Colin Wilson, and Carrie Beck. Directors include Filoni, Steph Green, Peter Ramsey, Jennifer Getzinger, Geeta Vasant Patel, and Rick Famuyiwa.
Seven of eight episodes of Ahsoka are now streaming on Disney+.