The Holy Grail[2] was a chalice owned by the Chiss Grand Admiral Thrawn in 2 BBY. It was stored in his office aboard the Chimaera.[1]
Behind the scenes
In real world theology, the Holy Grail was the cup used by Jesus Christ during the Last Supper. The Holy Grail's appearance in Star Wars Rebels was a reference to the 1989 film Indiana Jones and the Last Crusade, in which the Holy Grail was featured as the MacGuffin.[2]
Appearances
Star Wars Rebels — "Through Imperial Eyes" (First appearance)
Sources
"Through Imperial Eyes" Episode Guide | Star Wars Rebels on StarWars.com (backup link) (First identified as Holy Grail)