The Vow of Yunobo, Sage of Fire is a Key Item in Tears of the Kingdom that is used to summon or dismiss the avatar of Yunobo.
It is obtained after completing the Yunobo of Goron City 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 Yunobo that increases his avatar's attack power.
The avatar of Yunobo can't be summoned in the Depths beneath Hyrule Castle and in towns and villages.