Each Boss Soul can only be obtained once per playthrough, meaning you will need to beat the game multiple times to obtain every possible reward.
To open up the possibility of using Demon Boss Souls to make new weapons, you must first use the Flamelurker‘s Searing Demon’s Soul by giving it to Blacksmith Ed to unlock Advanced Weapon Crafting. Many of these new crafting recipes will require other powerful 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 the Boss Souls in Demon’s Souls. Click on a soul to learn more information on the various uses they can have.