User blog comment:Ornsteinthedarkhorse/Four Knights of Gwyn/@comment-25565800-20141127032311/@comment-4511886-20141226171003

The "fairy dust" deals magic damage and appears very similar to the effect on the screen when Prisilla turns invisible. Invisibility and magic qualities are shared with the crystals in Crystal Cave, so the dust is more than likely ground-up crystals. It's possibly a trace of Seath's powers, as his breath also deals magic damage and creates crystals.