I love the backstory, but would be keener to have the magical effect be more subtle and music related:
Otherwise, nice item! I like that the horn gets its own legend, and that bards make song in an attempt to attract it.
Very enjoyable read. I really liked the Lore part and how it's design is to improve the luck of the impoverished kind. I think it would be interesting to further design a full set of drawbacks for those not worthy to use Silverbreath and perhaps more of examples of the benefits. Good work.
Is the drawback really a drawback?
Being a fan of subtle magics, I'd prefer the horn to be just luck-fortifying, more so as I cannot find why poisonous or molten hot mists would be useful to a simple, however brave, shepherd. The refreshing/healing property fits quite well though.