Rune of Metamorphosis

Rune of Metamorphosis is a trinket for druids that can temporarily remove their mana cost of shapeshifting.

Source
Rune of Metamorphosis drops off Vaelastrasz the Corrupt in Blackwing Lair.

Trivia

 * Rune of Metamorphosis is an Uncommon Trinket card in the World of Warcraft: Trading Card Game's Drums of War set.