The Grand Admirals were some of the most powerful people in the Empire. After Emperor Palpatine's death, many of them died after they turned on each other or were eliminated by the New Republic.

Years after the Battle of Bilbringi, Admiral Pellaeon along with his fleet including the Super Star Destroyer Reaper arrived at the world known as Elysium. While the Grand Moffs at Bastion continue to bicker, Pellaeon traveled here to find a way to defeat the New Republic. According to Thrawn, he found this planet while exploring the Unknown Regions and named it Elysium. The inhabitants were apparently friends of Thrawn and were referenced many times in Thrawn's journal as important to the Empire.

What makes this planet so important that Thrawn would list the planet as "Our last hope"?

Warning: The Star Wars Universe is heavily altered. If you don't like that, then simply don't read. I do not appreciate people who judge stories simply by reading the description.