When the Extended Universe of Star Wars was snuffed out, fans cried out in anguish - they lost one of their favorite villains, Grand Admiral Thrawn of the Heir to the Empire books. A blue skinned baddie with some kind of Force-dampening lizards (I'm typing this in a convention hall from memory of books I read decades ago), he has been the one "Legends" character who fans have long been clamoring to become canon. And now that is happening in Star Wars Rebels season three.
In the aftermath of the vanquishing of the Inquisitors in season two and the continued successes of the nascent Rebellion, Grand Moff Tarkin has reached out to someone more likely to get results. And from the trailer we saw it seems that Thrawn starts getting results right away, making life hard for the rebels.
This should be interesting - Thrawn is the only non-human Imperial officer, so hopefully we get a sense of why Thrawn has been able to make it to the level he is at. That level, by the way, is Imperial Admiral, not Grand Admiral. For now.
Lars Mikkelsen - brother of Rogue One star Mads - will be voicing Thrawn on the show, debuting this fall on Disney XD.