Arch-Mage Shalidor was an immensely powerful wizard in the First Era who had an understanding of magic few have ever matched, and his numerous but scattered writings are highly sought after. There are many legends surrounding Shalidor's life, such as how he built the city of Winterhold with a whispered spell, how he stole the secret of life from Akatosh, and how he single-handedly fought against the Dwemer legions of clan Rourken at the Battle of Rourken-Shalidor (in which the Spell Breaker is said to have been used). Many of these stories are believed to be hyperbole or outright fabrications. Shalidor stood at the forefront of a movement to enact higher standards among mages, and to discourage spell use among the common castes. This effort is dubiously credited with the original organization and formation of the schools of magic (Vanus Galerion is more often credited with doing this) and the foundation of the College at Winterhold.
He holed himself up in Labyrinthian, where he constructed the namesake labyrinth to test new archmages, and devoted all of his time to research and study. He became an incredibly prolific writer, and scholars have worked to track down his notes and musings for thousands of years. His great maze is said to hold "Glamoril", or the secret of life in elvish, which, as noted above, was said to have been stolen from Akatosh by Shalidor. There's some speculation that this secret of life he possessed, whatever it was, may have been the catalyst for the practically unbelievable number of writings he produced on a vast array of topics.
Shalidor is also credited for establishing a sanctuary for all mages across the ages on the island of Eyevea, an outlier island in the Summerset archipelago which he obtained from the High Elves in exchange for some minor artifacts. He bargained the island away to Sheogorath in exchange for the Folium Discognitum, a bargain which he later came to regret. His obsession to recover Eyevea from Sheogorath occupied him so much as to cost him the love of his wife, Ulfsild.
- For game-specific information, see the ESO article.