The overall storyline of Marvel’s Phase 1, 2 and 3 has been the discovery of various Infinity Stones, objects of extreme power in the Marvel Universe. We know that Thanos is searching for them, and we know where all but one is. The question is, where is the Soul Gem and when will we see it?
Of all the Infinity Stones in the MCU (which are slightly different from their comics counterparts) only one is not accounted for.
The Space Stone is currently safeguarded on Asgard. The Mind Stone currently resides in Vision’s forehead. The Reality Stone is held by Taneleer Tivan, the Collector on Nowhere. The Power Stone is currently held by the Nova Corps. The Time Stone is held within the Eye of Agamotto in the hands of Doctor Strange.
So this begs the question: Where is the Soul Stone?
The Soul Stone is typically held by Adam Warlock (when it’s not being wielded by Thanos, that is). At the end of Guardians of the Galaxy Volume 2 we learn that Aeysha, the Sovereign High Priestess is creating a new form of life named Adam. Let us put speculation aside for a moment and declare that this is Adam Warlock, who will (assuming the story follows its comic inspiration) play a major role in Avengers: Infinity War.
However, I don’t think that’s where we’ll first see the Soul Gem. I believe we’ll see it in Thor: Ragnarok which appears actually to be a remake of the Planet Hulk comic series. In the trailer for Ragnarok we see that The Grandmaster is overseeing the games that Thor takes place in. In the comics the Grandmaster has typically held the Mind Gem, but we know this is held by Vision.
One possibility is that The Grandmaster holds the Soul Stone, which has the power to steal people’s souls, raise them from the dead and can even manipulate people’s souls. This is slightly in conflict with the Mind Stone, which in the MCU controls people’s minds but not in the comics universe. Still, it is conceivable that the Grandmaster is using the Soul Stone to control his combatants.
But, there is another possibility. That of Hela. In the comics, Hela holds a pact with Death (she is a death deity) and can steal people’s souls and has control over life and death. It seems to me that the likely plotline for Hela in Ragnarok is that she was the original wielder of the Soul Stone, which granted her these power. We know that Hela is released from some sort of prison (somehow) and seeks vengeance against Asgard.
Whichever possibility is true (I personally believe Hela will wield the Soul Stone) it is more than likely that the Soul Stone will be introduced in Thor: Ragnarok which leaves Thanos free in Infinity War to collect them rather than simply search for them.