Magic spells were essentially descriptions of Laws, like theoretical formulas that had been summed up by people. The mages below the Legendary-realm were still unable to really comprehend the mystery of the Laws, and could only use magic spells to activate and mobilize the nomological power in order to cast magic. Putting aside the Sanctuary powerhouses that were almost godly, even the ones in the Legendary-realm were almost free from relying on magic spells.
The Immortal King was a powerful existence that was probably not at all inferior to actual gods. The Immortal King's mastery of the Laws was already beyond just comprehension and simple control. In fact, he had already reached the level of being able to change existing Laws and create new ones. Given the Immortal King's level, how could he solemnly write the Book of Ten Thousand Incantations? Surely he couldn't have been thinking of using it as teaching material for the High Elves who were beginners in magic?
