Each Soul Boss can only be obtained once per game, which means you will have to beat the game multiple times to get all the possible rewards.
To open the possibility of using Demonic Boss Souls make something new weapons, you must first use the Flamelurkerof Soul of the Burning Demon by giving it to Blacksmith Ed to unlock Advanced Weapon Crafting. Many of these new crafting recipes will require additional weapons in addition to the boss soul to create something new – but they can also be given to spellcasting NPCs to learn new spells.
Below is a list of all Souls bosses in Demon’s Souls. Click on a soul to learn more about the different uses it can have.