The blowback spell was a spell from the Grimorum Arcanorum.
The Magus used this spell against the Weird Sisters when they tried to turn him Princess Katharine, Tom, Mary and Finella into owls. He was not only able to block their spell but turn it against them, changing them into owls. ("Avalon" Part One)
- Per iussum meum
- Fierateres ut optare