Pixie Dust

Pixie dust is a magical substance created by Pixies. It may have many magical properties, but so far it's only known to be able to control the Sidhe inside a Changeling. When Pixies are killed, probably only by powerful Sidhe magic, they are reduced to Pixie dust. This happened to Grunhilda, who was killed by Merlin with the use of his Sidhe staff (The Changeling).