Magical mottoes are the names used by magicians in the black magicsociety or the names they use when exercising black magic. Magical mottoes are chosen by the magicians themselves as pledge to "what kind of magician they want to be" and they bear the duty of being a magician worthy of that name. By thoroughly deepening the magical comprehension of one's magical motto, they earn the right to gain another magical motto of a higher, meaning excellent magicians carry multiple magical mottoes. Additionally, it's said that the order that spread the practice of renewing magical mottoes was Aleister Crowley's order (Argentum Astrum) and said order is credited for having established the ranking system for the magical mottoes' grades.
To Mega Therion (Greek for "The Great Beast") is one of Aleister Crowley's magical mottoes. Incidentally, another one of his magical mottoes was Baphomet.