One-Winged Angel/Oral Tradition

As a heroic example, in the epic, Táin Bó Cúailnge, Cuchulainn enters a "Ríastrad" or "Warp Spasm". In this state he transforms into a horribly mutilated monster whose very appearance is enough to scare enemies away. The catch is that, while in this form, Cuchulainn may not know friend from foe.
In The Kalevala, the evil sorceress Louhi turns into the mythical thunder bird Kokko to pursue the heroes, wielding scythes for her claws (Older Than Radio).
A lot of Hindu deities have avatars that basically serve this purpose. Kali, in particular, is a terrifying and destructive (but not evil) form the warrior-goddess Durga assumes to deal with the extremely powerful demon Raktavija. It doesn't end well.
Jesus, in the Book of Revelation - particularly chapter 19, where he shows up as a Warrior Prince with flaming eyes, surrounded by Instant Runes and riding a white horse at the head of a legion of angels. He spits a burning sword out of his mouth, summons an army of carrion birds, and proceeds to lay the smack down on the various monsters, demons, and false prophets that had been corrupting the earth and fighting with his forces for the previous couple chapters.