Thrawn's Revenge is a large-scale mod for Empire at War: Forces of Corruption. It includes the events of the Galactic Civil War after Palpatine's death at Endor, up until the signing of the Pellaeon-Gavrisom treaty. If you own the Steam version of EaW, you will have to complete extra steps to run the mod after installation. Please see the manual for these steps.
The Kossak-classfrigate was a design introduced in Lords of Nal hutta, an RPG sourcebook, as a small vessel created for the Hutts by Ubrikkian Industries. Valerie has taken the vessel, which never received a proper design and given it a brand new look for the Hutt faction in Thrawn's Revenge.