Once u have a fragrant branch of yore u can find the npc in lost Bastille unpetrify him and trade boss souls for weapons.
Now for a question of my own, since it's related.
Is he the only NPC in the game that takes boss souls? If he has nothing that interests me I can use that soul with impunity right?
Ornifex also trades boss souls for weapons, and is the only one that you can trade some of the boss souls to. To reach her, you need another fragrant branch of yore, and need to use it on the petrified lion enemy next to the third bonfire in the shaded woods (the one right across the tree bridge from the bonfire). That enemy will drop a key that unlocks Ornifex's room, which is hidden below a breakable wooden platform to the right of the giant frog. Drop down, kill the regular frogs, and unlock the door. She'll tell you to meet her at her home in Brighstone Cove. You can't really miss her there, as her home is at the bottom of the pit next to the quicksand.