It was revealed earlier this year that Tilda Swinton will be in the highly anticipated Avengers: Endgame. This is exciting, yet confusing news since her character died in 2016's Doctor Strange. However, the main theory floating around about Endgame is that the use of time travel, perhaps through the mysterious Quantum Realm, will play a large role in the sequel as Earth's Mightiest Heroes attempt to reverse the effects of the Mad Titan's Decimation from the conclusion of Infinity War.
The addition of Tilda Swinton's Ancient One was revealed by Avengers: Endgame executive producer Michael Grillo, who said, "When we got Tilda Swinton, she was just a one-day availability." Additionally, the actress is listed in the IMDb page for the sequel. In Doctor Strange, it is revealed that the Ancient One is the guardian of the Eye of Agamotto, which contains the Time Stone. In addition to protecting it, she added that she "spent so many years peering through time" and has "prevented countless terrible futures," which would leave one to believe that she is using the Infinity Stone to do so.
Like Doctor Strange in Infinity War, the Ancient One has more than likely seen the future and Strange's place in it. She could be a powerful ally in the reversal of Thanos' Decimation if the Avengers use the time travel to go back to a time when she was still alive. But, how will they know to go back to visit her? That may all end up on the shoulders of Doctor Stephen Strange, who will reportedly be talking an awful lot about the Quantum Realm, according to notorious blabber mouth Tom Holland.
In an interview promoting Infinity War last year, Tom Holland revealed that Benedict Cumberbatch had the toughest lines to remember because he has to talk about the Quantum Realm a lot. Strange does not mention the mysterious realm from Ant-Man and the Wasp in the last movie, which certainly means that he will be discussing it in Avengers: Endgame. But, how will this factor into seeing the Ancient One on the big screen again? With only one day of filming, it's hard to believe that she will have lengthy scenes in the movie.
As with most things in the Marvel Cinematic Universe, the details about Avengers: Endgame are being treated with intense secrecy. The Russo Brothers and Marvel Studios have been taking extraordinary precautions to make sure that nothing leaks, going as far as to only give the actors in the movie short little bits of the script, keeping them in the dark as much as possible. The actors were all unaware of the official title, along with when the first trailer was going to drop. But, sometimes things find their way online, like Tilda Swinton reprising her role as the Ancient One. You can head over to IMDb to see Swinton's name on the cast list.