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."

- Red Skull 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 Red Skull, before Thanos came to the planet in search of it and had to tragically 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 Red Skull
"What you seek lies in front of you. As does what you fear."

- Red Skull 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, become a Stonekeeper and serving as an adviser 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 Red Skull 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.

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.

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.

Following the Decimation, Thanos retreated to the utopian world of Titan II where, after three weeks, he turned the power of the Infinity Stones inward, destroying them. In 2023, the Avengers time travelled to collect past versions of the Infinity Stones and use them to undo the Decimation. In 2014, Hawkeye and Black Widow travelled to Vormir where the latter sacrificed herself to allow Clint to claim the stone. When Bruce Banner later enacted the Reversal of the Decimation, he attempted to resurrect Romanoff but was unable to owing to the rules of the Soul Stone. Captain America later returned the Soul Stone, along with the others, back to its proper place in time.

Capabilities
The Soul Stone ranks as possibly the most dangerous of all the Infinity Stones, with it 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. The Soul Stone has the ability to manipulate the soul, the essence that makes up an individual, and has the ability to resurrect and conjure the spiritual representation of the people who are dead.

Upon Thanos acquiring it, the Titan uses it the least out of all the Stones but still displays familiarity with the Soul Stone's powers. During the Battle of Titan, when cornered by Doctor Strange's mirror images, Thanos 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 briefly forcing Strange's astral form out of his body. 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 Thanos' decimation. The Soul Stone contains a pocket dimension, consisting entirely of shallow water underneath a dim orange sky, where the souls of the dead can be communicated with.

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.