What are the requirements to craft Demon weapons? Or weapons that require a specific Demon's soul to make?

Like I'm trying to craft the curved sword with Soul of Quelaag, and I've tried using both a +10 Scimitar and a +10 Laito/Iaito, and neither seem to work. Do I need a specific Ember? A specific Blacksmith? I've tried bot the blacksmith outside Sen's Fortress and the one in the ruins under Firelink (with the ghosts).

I'm not sure what the sword is called, it's gone by a few names, like Quelaag's Fury Sword, Chaos Sword, etc, but every source I've seen says it requires a +10 Curved Sword and Soul of Quelaag. Whether it means Katana or just a generic Curved Sword, I've got a +10 of both. So what am I missing?

Hey thursdae, what blacksmith are you going to? I believe only the behemoth blacksmith in anor londo can use the boss souls and +10 weapons to create the desired weapon.

For unique boss soul weapons, the giant blacksmith at Anor Londo does them.

You can craft Quelaag's Furysword from a +10 scimitar/falchion. I did it with a falchion.

Um....you can't really miss him if you full access to Anor Londo. He's the opposite side of where the third bonfire, the one I presume you are using as a checkpoint to the Anor Londo boss. So instead of going down to where the boss is, go up and go round.

yes, thats the one I'm referring to. Its actually the third bonfire but anyway, like I said instead of heading down to the main hall of the church where the two giant knights are, head up and around.

Oh yes, the Crystal Shield is actually very bad, as I had the luxury of discovering too :/

Don't roll, enraged Ornstein's attacks are all blockable in the case of which he opens up the distance between you and him.