The Avengers' time heist in Avengers: Endgame saw several past stories being revisited, including 2013's Thor: The Dark World, 2014's Guardians of the Galaxy, and 2012's epic crossover event, The Avengers. In this way, Endgame acted as a celebration of the MCU until that point, with the six original Avengers leading the charge on the mission. Endgame also featured the return of several past MCU characters, such as a previously-recorded cameo from Jane Foster, the return of the Red Skull, and an appearance from Doctor Strange's Ancient One, portrayed by Tilda Swinton, who provided the Hulk with some important information that he seems to have forgotten about.
The Ancient One Warned Hulk About A World Without Infinity Stones
Traveling back to 2012, Iron Man and Ant-Man set out to acquire the Tesseract, a.k.a. the Space Stone, while Captain America collected Loki's scepter, housing the Mind Stone, from the HYDRA-affiliated STRIKE team, in the wake of the Battle of New York. Bruce Banner, however, now in his Smart Hulk form, met with the Ancient One atop the Sanctum Sanctorum in an attempt to gather the Time Stone. Understandably, the Ancient One was initially reluctant to hand over the Infinity Stone, and informed the Hulk that should the stones not be returned to protect their home timelines, the forces of darkness would be unleashed.While the Hulk eventually persuaded the Ancient One that the Infinity Stones would all be returned, he forgot one major event that occurred five years earlier. The opening moments of Avengers: Endgame detailed the Avengers setting out to find Thanos and reverse his snap only three weeks after his attack in Wakanda, but find the Mad Titan alone and weak after having used the Infinity Stones to destroy the Infinity Stones. This means that, in the main MCU continuity of the 616-universe, the Infinity Stones have been absent for several years now, but the Hulk has neglected to pass on the Ancient One's warning of reality collapsing without the stones.
