Fairy Powder

From Zelda Wiki.org

Fairy Powder is an item obtained in Oracle of Ages. Upon discovering that the seas are being poisoned, sickening King Zora and creating obstacles to Link's progress, he sets out to find a way to solve the problem. A Fairy Queen who has been turned into an Octorok tells Link that she can be cured with Fairy Powder, which is kept in the nearby library. Using the Harp of Ages to travel between the past and present ages, Link is able to procure the powder from the library, and subsequently use it to cure the fairy, who cleanses the seas with her magic.