Warning: SPOILERS for Thor: Ragnarok are ahead!
Five Infinity Stones have been accounted for in the Marvel Cinematic Universe, leaving only one left to be properly featured: the Soul Stone, which has so far only been briefly glimpsed during The Collector's history lesson in Guardians of the Galaxy. Some speculated that the final Stone would pop up in Thor: Ragnarok (specifically in Heimdall or Odin's possession), but that theory proved to be incorrect. There are still plenty of hiding places for the Soul Stone, and with Black Panther and Avengers: Infinity War as the only movies left to introduce it, we'll find out its location in the coming months. However, it's worth considering that rather than still being secretly tucked away, the Soul Stone is already in Thanos' possession, and he'll use its power when he finally comes to the forefront in Infinity War.
Let's look at Thanos' history with the Infinity Stones in the MCU so far. In The Avengers, he loaned the the Mind Stone to Loki so that he could retrieve the Tesseract, a.k.a. the Space Stone, which had been hidden on Earth for centuries. At the time, we didn't know that Loki's staff contained an Infinity Stone, so in hindsight, the Mad Titan lending this artifact Asgardian god of mischief was particularly risky. That risk ultimately didn't pay off, as not only did Loki not deliver the Tesseract, but his scepter was also left behind on Earth and eventually experimented on by HYDRA. Two years later in Guardians of the Galaxy, Thanos promised to lend his forces to Ronan the Accuser so the Kree fanatic could wipe out Xandar in exchange for Roanan locating the Orb, a.k.a. the Power Stone. Once again, Thanos' faith in an underling/ally was misplaced, as Ronan decided to use the Power Stone for himself, and the purple glowing rock ultimately ended up with the Nova Corps. By Avengers: Age of Ultron, Thanos decided enough was enough, and now he's off searching for the Infinity Stones himself.
Which bring us to the Soul Stone. While it's possible that the final Stone is still resting comfortably somewhere, be it on Earth (which had already been home to the Space and Time Stones before the modern MCU kicked off), another planet or even another dimension, we suspect that it's already in Thanos' hands. If this is the case, then how long has he had it? There are two primary options. One, he found it before the events of The Avengers, be it thousands of years ago or only a couple days before the movie. Lending the Mind Stone to Loki was still an incredibly risky move that backfired in the end, but if we assume Thanos had another Infinity Stone in his possession, it comes off as more of a calculated risk.
The other option is that Thanos obtained sometime after he donned the Infinity Gauntlet in Avengers: Age of Ultron. If the Age of Ultron mid-credits scene takes place shortly after the movie's main events, that means by the time Thor: Ragnarok unfolds, it's been two years, and we doubt that Thanos has been twiddling his thumbs in that period. With the help of The Black Order, perhaps he found the Soul Stone's location and retrieved it without much trouble. So by the time he and his minions arrive in Avengers: Infinity War, one of the Infinity Gauntlet's slots will already be filled.
However Thanos may have acquired the Soul Stone (and keep in mind that we may never learn where it was originally located), you can count on him boasting incredible power with it, even compared to the other Infinity Stones. In the comics, the Soul Gem allows its users to manipulate to steal and manipulate both living and dead souls, as well as steal powers from others. Essentially, this Gem allows whoever holds it it to control all life in the universe. The MCU may decided to alter the Stone's capabilities, but regardless, Thanos will have a significant edge over his enemies, and that's saying a lot for a guy who is one of the strongest beings in the universe and is aided by the mighty Black Order, a.k.a. his "children" who haven't escaped his grasp like Gamora and Nebula.
As it stands now, the only Infinity Stones that are with the MCU superheroes are the Mind Stone (which powers The Vision) and the Time Stone (which Doctor Strange can grab if necessary). The Power Stone is still with the Nova Corps, but it's reasonable to assume it will be taken from them soon; the Reality Stone is presumably still with The Collector, having survived the destruction of his museum; and while it wasn't directly shown in Thor: Ragnarok, we know that Loki stole the Space Stone before Asgard was annihilated by Surtur (and he was seen handing it to someone in the Avengers: Infinity War D23 footage). Our esteemed heroes will fight their hardest to make sure that Thanos doesn't seize all six Infinity Stones and obtain ultimate power, but if he's already armed with the Soul Stone, their jobs became much harder.
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The Marvel Cinematic Universe will continue with Black Panther on February 16, 2018, and Thanos will finally prove why he's the big bad of the entire MCU when Avengers: Infinity War is released on May 4, 2018. We also shouldn't discount that the Infinity Stone search could continue into Avengers 4, which opens on May 3, 2019. Keep checking back with CinemaBlend for more news on Thanos and the rest of the MCU franchise, and don't forget to browse through our 2017 premiere guide and 2018 premiere guide.