The Soul Stone, which has the ability to restore, take away and control all life in the universe, appeared in the comics on the forehead of the cosmic hero Adam Warlock, who appeared at the end of Guardians of the Galaxy Vol. 2. Unfortunately, director James Gunn has confirmed that we won't see Warlock again until Guardians of the Galaxy 3, plus we've already got a character in the MCU with an Infinity Stone on their forehead, so adding another would feel redundant.
Considering how little we know about the Soul Stones, many fans have done what they do best: make fan theories. Here are best theories so far about the Soul Stone's location:
The Eternal Flame.
In Thor Ragnarok, we got a good look at one of Asgard's most powerful artifacts: the Eternal Flame. The Flame, as its namesake describes, can never die. It also has the ability to raise the dead and give life, as we saw it do to Hela's army and to Surtur. This and its orange color have led many fans to believe that the Eternal Flame may be the Soul Stone. Even though the Flame was seemingly destroyed during the obliteration of Asgard, the fact that it is "Eternal" may have still kept the Flame alive.
The strongest reasoning against this theory, unfortunately, is that at the end of Thor: The Dark World, the Warriors Three gave the Collector the Reality Stone because they claimed one Infinity Stone on Asgard was enough. The "one Infinity Stone" they spoke of was the Space Stone (The Tesseract), which was delivered to Asgard at the end of The Avengers. However, if the people of Asgard never realized that the Eternal Flame was the Soul Stone, this bit of dialogue could be overlooked, proving this theory to be rather feasible.
Heimdall has it.
Heimdall, the former gatekeeper of Asgard, is the subject of another Soul Stone theory. Heimdall can see and watch over all of the souls across the Nine (now eight) Realms, which has led people to believe he has, is, or has some connection to the Soul Stone itself. Additionally, Heimdall's eyes are an unnatural orange, which is the same color as the Soul Stone, which only helps to add to this theory.
This theory was ultimately backed up when Heimdall's actor, Idris Elba, was listed on the Infinity War poster's credit block. In all honesty, this theory is pretty plausible. I wouldn't be surprised if, in Infinity War, Heimdall ended up owning the Stone, having the Stone inside of him, or at the very least knowing the Stone's location.
Tony Stark is The Soul Stone.
In Iron Man 2, we learned more about the relationship between Tony Stark and his father Howard. During a video to Tony from his father, Howard tells Tony, "What is and always will be my greatest creation is you." People have theorized that, given Howard's connection to S.H.I.E.L.D., who may have gotten hold of the Soul Stone at some point, Howard used the Soul Stone to create Tony. Fans theorize that Howard did this because neither he nor his wife were able to have a child, due to their age around the time Tony would've been born. Howard instead used an Infinity Stone to create life. If this is true, the apple wouldn't have fallen far from the tree, considering Tony did the same thing twice in Avengers: Age of Ultron when he created Ultron and Vision.
While this theory was initially implausible, it was recently backed up by some promotional artwork from Avengers: Infinity War. The artwork was made in six parts, each dedicated to one Infinity Stone. Within each Infinity Stone was a character that had something to do with it (Star Lord was with the Power Stone, Doctor Strange was with the Time Stone, etc.). The only character with an Infinity Stone that didn't make sense was the image of Iron Man within the Soul Stone. Now that we know that there is a connection between the Stone and Tony, the theory of Tony being the Soul Stone starts to make sense.
It's in Norway.
Toward the beginning of Thor: Ragnarok, Thor and Loki travelled to Norway to find, and ultimately say goodbye, to their father Odin. During their conversation with the late King of Asgard, Odin told them both to "remember this place." He then disappeared in a glimmer of orange.
Fans have theorized that this is the location of the Soul Stone, explaining how Odin's soul was so easily lost. The orange glimmer that Odin became further backed up this theory, considering that orange is the color of the Soul Stone. It's a very interesting theory that would explain why Odin treated that part of Norway as such a holy place.
It's back In time.
Another theory of the Soul Stone is that it's location isn't about "where," but more so about "when." The Infinity War prelude comics briefly mentioned that the Soul Stone has supposedly not been seen in ages, which may explain why it has yet to appear in the films.
In order to retrieve the Soul Stone, Thanos first needs to retrieve the Time Stone from Doctor Strange. This would explain why the Time Stone is apparently one of the first Infinity Stones Thanos goes after in Infinity War. Additionally, adding a time travel element to the Avengers films would be a great way to spice up the series.
While some of these theories are more believable than others, they all may have a bit of truth to them. Unfortunately, we really know nothing about the Soul Stone's location, so watching previous films and studying promotional artwork on Reddit may be all we have to go off of. Which theory do you think is the most plausible?