Thrawn learned that Anakin Skywalker and Darth Vader were the same person during a mission with Vader, but kept the secret to himself. Thrawn's knowledge of Anakin's history explains his surprised reaction to hearing his name in Ahsoka episode 7 and likely gives him an advantage against Ahso...
saying that he's "really excited" for audiences to see his version of The Corinthian, an escaped Nightmare from The Dreaming with rows of teeth for eyes, who becomes a serial killer in the Waking World
actually somewhat defeated by Ahsoka in the season finale’. I think Disney’s portrayal of Vadar via Rebels shows a vastly flawed villain, who certainly does not compare to the heroics of Anakin Sky