Holy Grail

The Holy Grail was a cup which was owned by the Chiss male Grand Admiral Thrawn by 2 BBY. Stored at Thrawn's Office, the Grail was just part of Thrawn's artifacts collection.

Behind the scenes
In real world theology, the Holy Grail was the cup used by Jesus Christ, the son of God, in the Last Supper, which took place some days before his execution. The Holy Grail's appearance in Star Wars Rebels, however, was actually a reference to Indiana Jones and the Last Crusade, another Lucasfilm Ltd. film part of the Indiana Jones franchise in which the Holy Grail was the film's MacGuffin.