A frustrated Thanos then resolved to stop working through his proxies to power the Infinity Gauntlet after Loki lost the Mind Stone on Earth in a failed effort to recover the Space Stone. The Mad Titan's decision to get his hands dirty was cemented when Ronan the Accuser stole the Power Stone for himself in the original Guardians of the Galaxy.
Thanos continued to trust his Black Order to help him recover the Infinity Stones as part of his ongoing mission, but the Mad Titan himself would play a much more hands-on role in their recovery, actively participating in the attacks on the Asgardian survivors to acquire the Space Stone, thrashing the Collector's archives on Knowhere to claim the Reality Stone and venturing to Vormir with Gamora to obtain the Soul Stone.
At the end of Infinity War, Thanos faced the heroes gathered on Titan alone to take the Time Stone from Doctor Strange and, following the annihilation of the Black Order, snatched back the Mind Stone from Vision. What was formerly the first and only Infinity Stone in Thanos' possession years earlier became the final stone the villain obtained.
The retcon of Loki's scepter housing the Mind Stone is one of the most blatant and convenient retcons in the history of the MCU, on par with Hela offhandedly dismissing the Infinity Gauntlet in Odin's armory as a fake in Thor: Ragnarok. In-universe, it could be attributed to an early tactical error by Thanos, who underestimated the heroes of Earth and dismissed the possibility that they would ever effectively work together.
Loki would pay the ultimate price for his earlier failure at the hands of Thanos (literally), and the idea that the villain would ever part ways with an Infinity Stone before the completion of his plan is one that has never been addressed again.
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Tom Hiddleston, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Elizabeth Olsen, Chadwick Boseman, Sebastian Stan, Benedict Cumberbatch, Paul Bettany, Samuel L. Jackson, Cobie Smulders, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Pom Klementieff, Scarlett Johansson, Tom Holland and Anthony Mackie. The film arrives on April 26.