Here we take another look at the Infinity Stones, this time the Soul Stone / Gem. We've been going over the stones as Marvel 101's are doing the same and we've covered the Power, Mind, Reality and Time stones so far. The Soul Stone is a bit different than the others as it is sentient and has a desire to collect souls. It grants the user the ability to attack a soul and to defend their own soul from attack. It can gain information by peering into someone's soul. It can collect souls inside of the gem, placing the souls in a Heaven-like pocket reality, leaving their memories and skills accessible to the user. When used in conjunction with the other stones, the Soul Stone can access all life in the universe. The stone is mainly known for it's time with Adam Warlock, but has also been in the possession of the High Evolutionary, Gardener, In-Betweener, Thanos, Nebula, Count Abyss, Magus, Rune, Odeni, Gemini, Loki, Syphonn, Galactus, and Doctor Strange.
The Soul Stone has not appeared yet in the MCU, at least not officially. With James Gunn having said there would be not stone in the Guardians of the Galaxy Vol 2, it is most likely the final stone will be introduced in Thor: Ragnarok. Which plays into a couple of fan theories that are looking pretty good. One of which says that Hela is in possession of the stone which goes with the fact she is the Goddess of Death in Norse Mythology and appears in the film with enough power to break Mjolnir. It's then believed that Cate Blanchett's character would go on to substitute for Death in the Infinity War. The other theory doesn't contradict that other that Hela doesn't have the stone yet, that's why she's coming to Asgard for it. The theory is that Heimdall possesses the Soul Stone, which is how he is able to "see" everyone from his perch as gatekeeper of the Bi-Frost. His eyes glow orange, the color of the Soul Stone, and in the crazy dream Thor had in Avengers: Age of Ultron, Heimdall is blind. If this turns out to be true, it would mean that the Soul Stone was the first of the Infinity Stones seen in the MCU.