Soul Stone

"Soul holds a special place among the Infinity Stones. You might say, it is a certain wisdom." "Tell me what it needs." "To ensure that whoever possesses it understands its power, the stone demands a sacrifice." "Of what?" "In order to take the stone, you must lose that which you love. A soul for a soul."

- The Stonekeeper and Thanos

The Soul Stone is one of the six Infinity Stones, the remnant of a singularity that predates the universe. For some time, it was kept on the planet Vormir under the watchful eye of the Stonekeeper, before Thanos came to the planet in search of it and had to heart-breakingly sacrifice his beloved daughter Gamora to obtain it.

Creation
"There are relics that pre-date the universe itself. An ancient force of infinite destruction."

- Odin The Soul Stone is a remnant of one of the six singularities which existed before the universe's creation. After the universe came into existence, four cosmic beings known as the Cosmic Entities forged the energies of these singularities into six immensely powerful objects called the Infinity Stones. The Soul Stone represented the soul. It would later go on to cultivate a reputation of mystery throughout the galaxy. Even those who were well versed in the powers, and even locations, of the other five Stones had little concrete knowledge regarding the Soul Stone to the point of only knowing its name. Despite, or perhaps because of, this aura of mystery, it would eventually be considered as the most dangerous of all the Infinity Stones. Even the Asgardians knew little of the stone, with Thor stating no one had ever seen it or knew where it was.

Guarded by the Stonekeeper
"What you seek lies in front of you. As does what you fear."

- The Stonekeeper to Thanos The Soul Stone eventually found its way to Vormir where it became concealed in a shrine, accessible only to those who were willing to sacrifice someone that they loved. In 1945, the Space Stone banished the Red Skull to the planet after he had abused its power, due to knowing that the HYDRA commander would be unable to make the sacrifice required to obtain the Soul Stone. The Soul Stone then cursed Red Skull, transforming him into an immortal wraith with the power to immediately know whoever comes to Vormir and preventing him from leaving the planet. He eventually would go on to accept this fate, taking the alias of the "Stonekeeper" and serving as an advisor to those who would seek the Soul Stone's power.

As Thanos began his quest for the Stones, he sent the Black Order to scout the locations of the artifacts. Gamora and Nebula eventually tracked the Soul Stone down to Vormir only to conceal its location from Thanos with Gamora burning the map that had led them to it, though not before she had memorized its location. Thanos easily deduced that Gamora had located the Soul Stone, which he later attributed to her poor skill in lying, but allowed her to believe she had fooled him for the time being. Several years later, Thanos revealed to Gamora that he had never been fooled by her claims of never finding the Soul Stone and managed to force her to divulge the Soul Stone's location, via ruthlessly torturing Nebula, taking her with him to Vormir. As they neared the shrine, they were approached by the Stonekeeper who explained that Thanos had to sacrifice someone he loved in order to gain the Stone. Thanos then tearfully threw Gamora off of the cliff, her sacrifice allowing him to obtain the Stone after passing through the Soul Stone's pocket dimension.

Thanos' Use
Having successfully claimed the Soul Stone, Thanos returned to Titan, where he was confronted by an alliance of the Avengers, the Masters of the Mystic Arts and the Guardians of the Galaxy. When Doctor Strange cast a spell creating dozens of copies of himself in order to restrain him, Thanos used both the Soul Stone and the Power Stone to track down the real Strange and destroy the copies after the sorcerer had duplicated himself. And after he defeated Iron Man, thanos would soon prepare to use all four Stones that he had to execute Iron Man before Strange bartered the Time Stone for Stark's life.

Thanos then headed to Wakanda, where he retrieved the Mind Stone from Vision. Although he was stabbed by a vengeful Thor, Thanos retained enough strength to complete his quest, activating the Infinity Gauntlet with all the Infinity Stones, including the Soul Stone, to wipe out half of all life in the universe. This act briefly sent his consciousness into the Soulworld, where he briefly communed with a reflection of Gamora in her child form, telling her that he had achieved his goal but solemnly admitting that fulfilling his destiny had cost him everything.

Capabilities
As the Infinity Stone that governs over the fabric of soul the Soul Stone ranks as one of the most powerful artifacts in all creation. With it having the divine right to choose between who lives or who dies.The Soul Stone is considered to be the most dangerous of the Infinity Stones, holding a special standing among the others. Uniquely, in its shrine on Vormir, the Stone purposefully imposed a test to those who would try to claim it - only the sacrifice of a loved one to the Stone would allow for someone to wield it.

Upon Thanos acquiring it, due to the Soul Stone's far more unique and deadly powers, the Titan uses it the least out of all the Stones but still displays familiarity with the Soul Stone's powers. The Soul Stone allowed Thanos to detect and attack the souls of others, as when cornered by Doctor Strange's Images of Ikonn, Thanos swiftly activated the Soul Stone and combined it with the Power Stone to instantly destroy all the false images and reveal the real one while also spiritually attacking Strange, forcing his Astral Form out of his body for a brief moment. It also seemed to have granted Thanos the power to look into the souls of others to know all there is to know about them, apparently the same power it granted the Red Skull, as Thanos was able to tell Iron Man's true name and also knew that the Avenger bore the same "curse" as himself. The Soul Stone also contains a pocket dimension, consisting entirely of an infinite expanse of shallow water underneath a dim orange sky, where the souls of the sacrificed reside and can be communicated with. After Thanos used the Infinity Stones to eliminate half of all life in the universe, he briefly ventured into the world within the Soul Stone.

After the Infinity Gauntlet was completed, the Soul Stone was used in tandem with the five other Infinity Stones to wipe out half of all life in the universe with a snap of Thanos' fingers. The Soul Stone's specific role seems to be destroying the soul of the victims However, using all six Stones in this manner at such a weakened state caused the Stones's combined power to nearly overwhelm him and the Gauntlet as well as his left arm were left scorched by the release of energy.

Trivia

 * In the comics, the orange-colored (previously green) Soul Gem gives the owner the power to alter and manipulate all souls. It also acts as a gateway to its own pocket dimension. When used in conjunction with the other gems, it gives the user complete control over all life in the universe.
 * In The Superhero Squad Show, the Soul Gem has a pocket dimension for imprisoning others. It is also shown to manipulate emotions.