There's a sense in which Doctor Strange may be the most important Avenger of all - his knowledge of the Infinity Stones. Not only is he tremendously powerful in his own right, the future Sorcerer Supreme also wields the power of the Time Stone. He used that Infinity Stone to tremendous effect when he battled against the Dread Dormammu, and no doubt has continued to read from the Book of Cagliostro, learning more of the Time Stone's secrets. But, according to the official Avengers: Infinity War Prelude comic, there's another reason Strange will play a crucial role in Avengers: Infinity War.
Precious few heroes actually know about the Infinity Stones. Even those who've encountered them have failed to realize that the Infinity Stones are being brought into play; that cosmic forces are manipulating events to their own end, in order to acquire the Infinity Stones. Thor realized that in Avengers: Age of Ultron, and passed that news on to Captain America and Iron Man. But it turns out Doctor Strange is another hero who's in the know.
The Masters of the Mystic Arts Have Been Watching
The Prelude comic suggests that the Masters of the Mystic Arts know all about the Infinity Stones. Wong gives Strange a debriefing on cosmic events, revealing that Agamotto - the first Sorcerer Supreme - chose to use the Time Stone to defend our reality (something Stephen already knew from his own movie). The other Infinity Stones, however, were hidden; surprisingly, Wong even seems to know that the Power Stone was hidden on Morag, although he's unaware it was retrieved by Peter Quill. The depths of the Masters' knowledge is pretty incredible.
Aware of the threat of the Infinity Stones, the Masters have been watching recent events with concern. They know that the Red Skull took possession of the Tesseract in 1942. They even know S.H.I.E.L.D. retrieved the Tesseract, and attempted to weaponize it. They're also aware Loki used the Tesseract to bring the Chitauri to Earth, and that his scepter contained the Mind Stone. Wong knows that the Mind Stone was used to give Quicksilver and Scarlet Witch powers, and that it's now held by the Vision. Meanwhile, the Masters also know Malekith used the Reality Stone in Thor: The Dark World. That's particularly impressive, given only a brief part of the action took place on Earth.
The Masters of the Mystic Arts are sworn to protect our world. They've clearly been doing their job well, watching with care as they saw the Infinity Stones brought out into the open. Now, thanks to Wong's briefing, Doctor Strange knows the threat Earth faces.
Doctor Strange Will Play A Major Role In Infinity War
All this sets up Doctor Strange to play an important role in the third Avengers movie. So far, the trailer has suggested the heroes are somewhat divided for most of the film; the Avengers assemble on Earth, and the Guardians work with Thor in space. Both teams, however, has someone among them who knows what's going on with the Infinity Stones. Thor can tell the Guardians about them; Doctor Strange can debrief the Avengers.
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Ironically, though, Doctor Strange may well be a more significant player than even the God of Thunder. Unlike Thor, Strange has had time to master an Infinity Stone. He'd only read a few pages from the Book of Cagliostro when he used the Time Stone against Dormammu; given Strange still wears the Eye of Agamotto on occasion, he's no doubt been practicing. The trailer makes it clear that no Avenger can defeat Thanos when he has Infinity Stones in his grasp. But it's not certain the Mad Titan will acquire all of them. If the Avengers retain even a single Infinity Stone, Strange's knowledge could prove immeasurably valuable.
Doctor Strange knows that the Infinity Stones are being brought into play. He's also had the time, and arcane texts, to study them. That means, come Infinity War, Strange could actually be the world's most important defender. Thanos should beware.