It drops from those red Fire Knights in the Shadowkeep legacy dungeon, and since u said ur at Rellana or I'm assuming u ve beaten her, the next stop is straight there then
The best way to farm for it is the Storeroom First Floor in the dungeon, go down via the elevator where you originally came from to this grace and at the bottom there's always a fire knight ready to be backstabbed and killed from behind.
Rinse and repeat and fast travel back to farm until you get it