Boss Souls
In Dark Souls II bosses are the toughest enemies representing an area and need to be defeated to continue exploring. When defeated, a boss will drop souls for players to level up or buy, repair or upgrade gear. However, when a boss dies, the player will recieve the unique soul of that boss. This item can be used for souls or redeemed for a unique boss weapon.
Straid in The Lost Bastille must be unpetrified using a Fragrant Branch of Yore for him to trade with you, Straid trades boss souls for both weapons and spells.
Ornifex (found in the Shaded Ruins - relocates to Brightstone Cove) will also trade boss souls for weapons.