The Mystic Nine were a collective that was around during the High Republic Era. The existence of the Mystic Nine is connected to the Lariat of Armistice, an indestructible rope from the Jedi Archives which is dated to 231 BBY.
Cavan Scott included a reference to the Mystic Nine in his 2021 book The High Republic: The Rising Storm. This novel is a contribution to Phase I of the larger Star Wars: The High Republic initiative.