Fairy dust

is a magical powder generated from Fairies. Not much is known about it, but that it is able to put certain supernatural creatures to sleep such as Unicorns.

History
In "The Last Mabelcorn", Wendy, Candy and Grenda are given this by a mysterious gnome to take a lock of Celestabellebethabelle's hair after they exchanged it with butterflies that the gnome wanted. While the dust worked, they failed at acquiring the unicorn's hair as Mabel stops her friends and Celestabellebethabelle wakes up. Although they do ultimately retrieve a lock of unicorn's hair, the fairy dust had nothing to do with it.

Trivia

 * Fairy dust is probably the alternate version of Choloroform for supernatural creatures.