User blog comment:KodachiZero/Weekly News - 11/@comment-73.211.252.152-20160816203153/@comment-359947-20160816231750

I think those are used for forging the event weapons. There's usually a small chance of a prysma-colored weapon that will start you off with one mana. Not 100% sure for this though, they look different than the normal rusty weapons.