Doctor Strange secretly introduced the sorcerer who trained Doctor Doom - and subtly hinted that he may have rewritten the MCU's timeline once before. There's a sense in which Doctor Strange was one of the most important films in the MCU; it added a whole new aspect to the MCU: magic.

The star was Dr. Stephen Strange, who traveled to the secret temple of Kamar-Taj, where he was trained by the Ancient One and became a member of the Masters of the Mystic Arts. Blessed with a photographic memory, Strange proved a natural at sorcery, surprising his mentors. His curiosity also saw him find his way to some of the forbidden tomes of Kamar-Taj, most notably the Book of Cagliostro, which taught him the secrets of the Time Stone. In the real world, Count Alessandro Cagliostro was an 18th-century occultist who was known for psychic healing, alchemy, and scrying. In Marvel Comics, though, he was actually a powerful sorcerer who became interested in time travel after he encountered a couple of visitors from the future.

Related: Why Didn't Doctor Strange Trap Thanos In A Time Loop?

One of these was Doctor Doom, who had sought out Cagliostro as a mentor to help him learn sorcery. Just like the Ancient One in Doctor Strange, Cagliostro learned the secret of immortality - although in the comics it involved imbibing vampire blood rather than drawing on the power of the Dark Dimension. Again, like the movie, the comic book version of Cagliostro ultimately jotted down everything he had learned in a sacred tome, the Book of Cagliostro. It contained all of Cagliostro's own studies, some notes from a sinister book called the Darkhold, and even records from another time traveling sorcerer who had crossed Cagliostro's path. All in all, the MCU version of Cagliostro sounds very similar indeed to his comic book counterpart, right down to having the same preoccupation with time.

But there's one crucial difference. In Doctor Strange, the Eye of Agamotto is actually the Time Stone, and parts of the Book of Cagliostro are basically an instruction manual. When Strange attempted to practice the lessons contained within the Book of Cagliostro, he was interrupted by a horrified Mordo and Wong. They told him that temporal manipulations "can create branches in time. Unstable dimensional openings. Spatial paradoxes. Time loops!" As they explained, in the Book of Cagliostro the warnings come after the spells. Strange wasn't manipulating the space-time continuum - he was in danger of breaking it.

This, naturally, raises one interesting question: If Cagliostro learned - or more likely discovered - how to use the Time Stone, how did he find out how catastrophic the consequences could be? The only possible answer is that he learned through trial and error. Given the Masters of the Mystic Arts are sworn to protect the Time Stone, it's possible the MCU Cagliostro was a former Sorcerer Supreme, but wished to explore the Eye of Agamotto's abilities. Alternatively, as the Ancient One has been said to be 700 years old, he could have been one of her students who succeeded in stealing the Eye. Whatever the truth, in the MCU Cagliostro must have somehow acquired the Time Stone, and experimented with its power. He must have tried out at least most of the spells Doctor Strange was looking at, and experienced most of the consequences as well. It's reasonable to assume that Cagliostro actually did break the space-time continuum; perhaps time healed, or perhaps the Time Stone itself was used to repair the damage.

Whatever the case, the presence of the Book of Cagliostro - with its warnings noted after the spells - adds a fascinating extra dimension to the history of the MCU. It suggests that a powerful sorcerer once used an Infinity Stone liberally on Earth, experimenting with it in dangerous ways, and may have unwittingly changed the course of history as a result.

More: Time Travel In The MCU & Avengers: Endgame Explained

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