I'm kind of inclined to agree with Rune here, don't get me wrong here, its a rather fascinating idea though.

But looking at how the Souls games (Demon souls included, hell, even Bloodborne to a degree, granted, bosses didn't give souls to forge weapons into, they did sometimes give badges or guard them, which sort of functioned like that) have handled boss souls and how the player interacts with them, currency form of souls (Blood Echoes) included, forming either weapons or spells independent of the character in question, it doesn't seem to be something an undead would develop into, more organically mimicking the powers of dead enemies, both strengths and weaknesses. Definitely strikes me as a one off sort of rarity, NPC or even optional Boss in that regards.

I have no interest in following that path, personally, but the idea could make an interesting one off oddity of sorts. Definitely not something to be running around in, in large numbers, far as my opinion goes at least.