Thor, the Infinity Stones, and Thanos' role in Avengers: Infinity War
The last time we saw Thanos, back in the Avengers: Age of Ultron post-credits scene, he was putting on his Infinity Gauntlet, which had empty space for all of the cosmic Infinity Stones. Fans quickly pointed out that we'd actually seen the Gauntlet loaded up before, back in Odin’s treasure room in the first Thor movie. Ragnarok corrects that apparent error when Hela immediately knocks over the Gauntlet on Asgard and declares it a fake (which also makes sense as the Aether from Thor: The Dark World had to go to The Collector so it wouldn’t be in the same vault as the Tesseract, the other name for the Space stone). Hela does show great interest in the Tesseract, but then settles on… The Eternal Flame.
The Eternal Flame?!? If you’ve been following the reveals of the Infinity Stones a.k.a. Marvel’s magic MacGuffins, you know the Tesseract showed up in Avengers (as did Loki’s Scepter, though that’s not outed as a Stone until Age of Ultron), The Aether (the Reality Stone) appeared in Thor: The Dark World, The Orb showed up in Guardians of the Galaxy -- where the Collector finally told the backstory of the stones and gave us a glimpse at the unique colors for each one -- Ultron outed Loki's scepter as the Mind Stone (which the villain used to create Vision) and Doctor Strange (in Doctor Strange) slyly dropped that the Eye of Agamotto necklace was the Time Stone. That leaves one Stone left: The Soul Stone.
Many people assumed that the Soul Stone would be revealed in Thor: Ragnarok -- yours truly among them. It seems to fit so perfectly: the goddess of death Hela wanting a Soul Stone, Thanos wanting to court death in the storyline from the comics, even the acrostic theory -- where the objects hiding each of the stones spell out “THANOS” with the first letter of their noun (T for Tesseract, H, A for Aether, N for Necklace, O for Orb, S for Scepter) -- worked out. The “H,” us smart-asses thought, was for Heimdall, Idris Elba's all-seeing, bridge operator.