Found deep within Nokron, Eternal City (only accessible after defeating Radahn). See the above video at 4:03 for the path. You find it in a Chest right before Night’s Sacred Ground site of grace, behind Stonesword Key Gate. From Ancestral Woods Site of Grace in Nokron you must jump to the rooftops and parkour your way down.

Follow the ledges and roofs until you get into a cathedral building with an altar. Find the chest at the end of the hallway past the altar. It’s the building immediately before Night’s Sacred Ground site of grace.