The Vow of Sidon, Sage of Water is a Key Item in Tears of the Kingdom that is used to summon or dismiss the avatar of Sidon.

It is obtained after completing the Sidon of the zora questline.

After colecting 4 fragments of sage's will and talking to the Goddess Statue you can exchange them for an upgraded Solemn Vow of Sidon that increases his avatar's attack power.

The avatar of Sidon can't be summoned in the Depths beneath Hyrule Castle and in towns and villages.