Long before any of the Marvel Cinematic Universe’s heroes had the slightest inkling they might one day have to literally turn back time to save half of existence, Thanos was busy moving heaven and Titan to get his sizeable purple hands on each of the Infinity Stones.
In order for Thanos to become the ultimate villain, the MCU needed to round out its third phase, and his coming into the complete set of glittering MacGuffins was always a foregone conclusion. Without the threat of an Infinity Snap, there couldn’t have been Infinity War or Endgame. But before the Infinity Stones were brought together, each of them represented a distinct kind of power whose existences played into the ongoing events of the larger universe.
Even though they’re only at their full power when they’re used in concert with one another, each Infinity Stone was shown to be formidable in its own right, leading to Marvel’s heroes and villains becoming embroiled in a battle over them. This is true of all the Infinity Stones, save for the Soul Stone, which was finally revealed (after being shrouded in so much mystery) in Avengers: Infinity War.
Unlike the other stones that were introduced as dangerous tools which could easily be shaped into weapons in the wrong hands, the Soul Stone was unique in the way the film left its functional role and potential decidedly unclear. Once you drop what or whoever it is that you love the most down the Red Skull’s cliff on Vormir, the Soul Stone’s yours, and…that’s seemingly it.
In the Infinity War directors’ commentary, the Russo Brothers explain how, even though the film doesn’t explicitly state it, the MCU’s Soul Stone has some of the abilities its comics counterpart is known for, like conjuring the “spiritual representations” of the dead on another plane of existence.
In the time between Infinity War and Endgame, the Russos further elaborated on the Soul Stone’s powers by explaining how Thanos actually wields it briefly after using the Power Stone during his fight against Doctor Strange:
The Soul Stone obviously has the ability to manipulate your soul, the essence of who you are. One key moment where it’s used is where Doctor Strange turns into multiple Stranges and then Thanos uses the Soul Stone to eradicate all the fake Stranges and momentarily shoves Strange out of his own body, and Strange has to pull himself back in.
But as a plot device within Infinity War specifically, the Soul Stone was always meant to be the centrepiece of Thanos’ personal story of great loss on his path to annihilating the universe. Infinity War was Thanos’ movie, and at the moment when Thanos is forced to reckon with his willingness to kill the person he’s always loved most, the Soul Stone embodies all of the heartbreak and pain that would befall anyone foolish enough to hunt for the gems.
That framing of the Soul Stone worked to better characterise Thanos as a person, but it still left a rather large number of questions about the stone itself, and what (if any) role it would play in Endgame.
Given the stone’s connections to the dead, it made sense that it might factor into how the universe’s disappeared population might return to the land of the living, or perhaps provide a way for the MCU take on Lady Death to make her grand debut.
Whatever logical weirdness might have been associated with the Soul Stone — a fully-realised realm of the dead, zombies, Force ghosts, who knows — would have felt appropriate for Endgame’s life or death stakes. That’s what makes it so surprising and somewhat disappointing that ultimately, the orange rock didn’t end up doing all that much in the movie, aside from giving Black Widow a reason to plummet to her death.
Like many of Endgame’s other scenes set in the past, Black Widow and Hawkeye’s journey to Vormir is meant to be a nod to a pivotal MCU moment leading up to Thanos’ big Snap.
But rather than revealing anything new and fascinating about the world — the way Tony Stark’s time in the past with his father did, or Thor’s moment with his mother — the only thing we glean from the return to Vormir is that it’s awfully lucky that people keep bringing their loved ones there with them. What happens if you travel all the way to Vormir and leave your beloved somewhere else? What if the thing you love most isn’t a person or something that has a physical form?
When Thanos sees young Gamora again within the stone after his snap, she seems to be aware of just what her father’s done, implying that the stone itself is aware of what’s going on around it. But even that feels like small potatoes in terms of what the Soul Stone can do — compared to, say, the Reality Stone, which Thanos used to literally turn people into flesh ribbons.
The Soul Stone being the most conceptually complicated Infinity Stone is a nifty variable to work into the mix of things. But because so much of its true nature is only defined in vague subtext, there’s a way that it ends up also feeling like the least interesting of the stones, because the most important thing it does is exist.
Comparing the Soul Stone to Mati’s Heart Ring from Captain Planet almost feels appropriate until you consider that on more than one occasion, Mati demonstrated a number of active psionic powers.
But now that Thanos has been defeated and the Infinity Stones returned to their rightful places in the past, it feels as if the Soul Stone’s never going to have a chance to be handled like the same kind of MacGuffin its siblings were. That is, unless, it ends up being part of Adam Warlock’s whole deal in Guardians of the Galaxy Vol. 3.