Recondite was the personal Imperial-class Star Destroyer of Grand Admiral Nial Declann. It was presumably with him at the Battle of Endor.
Behind the scenes
Dan Wallace originally named the Recondite in a segment of his Grand Admirals article that was cut, and is therefore unofficial.
Recondite means hidden, and the ship was named to reflect Declann's moody nature.
Sources
"Who's Who: Imperial Grand Admirals" — Star Wars Insider 66 (First mentioned) (Cut content)
Grand Admirals: What Have They Got in their Pocketsses? on Continuity, Criticisms, and Captain Panaka — Daniel Wallace's StarWars.com Blog (original site is defunct)