How did Lance even master the Petrification Technique of the Medusa Clan?
If it were through chanting curses to display the Petrification Technique, that would be quite normal; many mages in the Human World could do that, so there would be nothing surprising about it.
But Lance was different; he petrified the Medusa Bessary with his eyes—this was a Pupil Skill!
The Innate Divine Ability of the Medusa Clan, the Petrification Technique, to be precise, is a Pupil Skill; it does not require any incantation, and can petrify a creature just by making eye contact.
Having mastered the Medusa Clan's Petrification Technique and successfully petrified Bessary through eye contact—if they had not seen it with their own eyes, they would never have believed Lance could attain such a feat.
According to their knowledge, the Medusa Clan should be immune to the Petrification Technique.
A Medusa like Bessary of her level should be even less likely to be petrified.
