Vow of Riju, Sage of Lightning

The is a Key Item in that is used to summon or dismiss the avatar of Riju. It is obtained after completing the Riju of Gerudo Town quest line.
Link may exchange four Sage's Wills at any Goddess Statue to upgrade the Vow into a Solemn Vow of Riju, Sage of Lightning, which grants the avatar increased attack power when assisting in battle and a slightly shorter ability recharge.
The avatar of Riju cannot be summoned in Beneath Hyrule Castle, Towns, and Stables.