The terrible Arcana of the "Silver Arche" that furthered both his madness and his power. The Arcana that named him Spell Thief hundreds of years ago. Nowadays considered nothing more than a silly urban legend at best.
Unknown, some drunkard at a bar might claim that The Beggar King claimed that he created it. But this arcana is considered to not exist at all, and only shows up mentioned in old, old, obscure and boring books that even the most bibliophile Azurite would have a hard time staying awake through.
Simply put; it lets Penn steal a minor Arcana from out of the very mind and soul of another person. This takes a long, long time of attunement and mind prodding in order to "break in" and grab whatever he needs to perform said spell. Little known fact is that it also lets him steal regular memories and even traits of personality from a person, something that cant be good for the mental health of the practitioner.