Grand Admiral Thrawn is an extremely powerful imperial commander, who can manipulate the turn meters of his units and thus strengthen them significantly. His acquisition is only possible during a very rare Artist of War event, which has no clearly defined time frame. The event consists of 7 levels, and on each of them you have to use a team of Phoenix Squadron units - they are the main heroes of the Star Wars: Rebels cartoon. The first fight requires at least 1 star in each character, the second fight requires 2 stars, etc.
The reward for completing each fight is a specific number of character shards, that are then used to unlock the unit. Important: the activation of Great Admiral Thrawn is only possible after collecting 145 fragments, over the course of 5 fights. Each skirmish can only be completed once, so it is only possible to promote the character to 7 stars after completing all stages. The character acquisition progress is saved, so even if you do not finish all the fights during the event, you can continue unlocking Thrawn the next time the Artist of War event is available.