The Soul Stone, much like the Reality Stone, latches onto its host and merges with their being entirely. It also has the ability to steal someones soul and leave that body lifeless. Unless the user is incredibly adept at wielding Infinity Stones the stone will suck the soul out of anything it touches. The stone even seems to have its own sentience, it has been known to speak, move on its own, and fire energy projectiles that assault the stone itself. On the small-scale the stone has the ability to fully understand any language; on the large-scale it can alter or destroy another soul. Those who are captured by the Soul Stone are taken to an inescapable realm inside the stone called Soul World.
In the comic books it was originally created when Nemesis destroyed itself. The Elders of the Universe bound all the Stones together in order to try to defeat Galactus. However, when the Elders ultimately fell, the Soul Stone resided with the In-Betweener. Thanos eventually obtained the Soul Stone during his conquest to obtain all the Infinity Stones, leading to the Infinity Wars.
In the Marvel Cinematic Universe (MCU) the Soul Stone has yet to be introduced. We know the stone is crucial for Doctor Strange’s story arc, so I am under the assumption we will see the introduction of the Soul Stone in his movie in 2016. If not then I can only assume then we will be getting the original introduction via Adam Warlock.
We have a glimpse of what could be Adam Warlocks cocoon during the end credits of Thor: The Dark World inside the Collector’s museum. If this is the case, then we may end up with the original origin of the Soul Stone, and have another super power on our side against Thanos.
It would be very cool to see Warlock show up in Guardians of the Galaxy Vol. 2. The problem is the Soul Stone has no business being on Earth at this point in time. The origin of the stone resides out there in space, so we will have to rely on a space based movie to bring our attention to its meaning. Therefore, either we get it in the next Guardians movie or we wait until Thor: Ragnarok.
Another bit of speculation is based around the Infinity Stones spelling out the name THANOS. Leaning on this theory can surmise that the Soul Stone resides with something that starts with the letter H. One of the common hypothesis is that the Stone resides with Heimdall in Asgard. This is less likely than Warlock, but still a distinct possibility.
Heimdall has the ability to see every single person in the 9 realms. With that as evidence, he also has an orange stone-like center piece in his armor, and orange glowing eyes to suggest that the stone may have combined itself with him. The other stones have the same effect when combining with their hosts; the Mind Stone causes a blue hue, Reality is a blackish color, and the Power Stone shows a purple glow. So if Heimdall has the Soul Stone built into his armor, and into himself, it would be consistent with the effects of the other stones.
***As the MCU progresses this page will be updated***