A pyromancy tome of the grave warden, from the Carthus catacombs.
Give to the pyromancer master to learn black flame sorceries.
High Lord Wolnir of Carthus succumbed to the Abyss, but the pyromancer later became a grave warden, and discovered the black flame.
Found in the Catacombs of Carthus. It is the item directly in front of High Lord Wolnir when you fight him. If you do not pick it up during the boss battle, it can be found on a corpse in the back left (facing the stairs to Izalith) of the boss room afterwards.