Uuugh. Given that it's poison from a fae, I'm not really surprised that souls are involved in this.

[She sounds super grumpy about this. Didn't they all have enough of soul talk back in Portland? Cripes.]

I know the Ringmaster has one of the Prince's fangs-- that's what the sword that Sans was using during your fight was, by the way. A manticore fang-- but I'm still rather unclear on its properties or how it works. Perhaps I can request to examine it more closely...