True magic is the original inherent magic of Lumen, its a fundamental overarching force that originates from deepwithin the earth of lumen. This force is deeply intertwined with the existence of Lumen itself.
True magic is significantly harder to learn than other powers in the world, compared to devil fruits true magic takes years of dedication or work to truly attain for the ungifted. Often its requires tools to channel and focus its power (staffs spell components etc).
When an indivisual transcends the need for tools and becomes capable of wielding the worlds original magic entirely themselves the power they wield becomes known as true magic. Only Full casters are capable of wielding True magic, they gain access to powerful unique spells, and the ability to create their own.