"Guardian of All Lawful Magic" is part of the ancient titulary of the Unicorn Kingdom and Old Equestria. It appears first in reference to Fancy Crown, crown prince of the unicorn kingdom, who was titled "hereditary guardian of all legitimate magic" at the time of his ascension of the Horned Throne of the Unicorn Kingdom in 298 BE.
The title was revived by the Harmonic Senate, and applied in its modern form to Princess Nova.