High Sorcery, also known as True Magic, is the practice of shaping the Wellspring into the form of spells, which can range from simple ones such as Prestidigitation to firestorms.
Following the Last Battle fought at the dawn of the Age of Shadows and the destruction of the capital of Lavellor, Magic, as both an institution and practice, declined and fell out of use. Most records were lost during the subsequent years of the War in the Dark, with the Great Fires of Sobsia destroying the Annex of the Grand Library situated in the city.
With the destruction of the Annex, and the various other incidents, the practice of True Magic slowly faded, being relegated to the curiosity of a later age, and the rare Aethercraft found through Rhaedwyn. The High Academy of Sorcery became the last institution in the continent studying it, focusing more on the philosophical and metaphysical nature of what magic is and what the Wellspring is, rather than practical application.
This has changed recently. With the discovery of the Shattered Reach and the reintroduction of many of the tomes kept by the Hamura peoples and their knowledge of the Lost Arts of Magic*, Rhaedwyn finds itself in a boom of Sorcery and magic, especially in the port cities such as Tanir, where contact with the Shattered Reach is much more prominent.
*The Lost Arts of Magic is what became known, in scholarly work, the loss of all practical magical manuals, and the refusal of the Sorcerer-King Aurozan of taking pupils.