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, heart-breakingly sacrificing 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. The Soul Stone would 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, simply knowing its name. Despite, or perhaps because of, this aura of mystery, the Soul Stone would eventually be considered as the most dangerous of all the Infinity Stones. Even the Asgardians knew little of the Soul 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, knowing the HYDRA commander would be unable to make the sacrifice required to obtain the Soul Stone. The Soul Stone then cursed the 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. The Skull 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.

In 2018, 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.

In Thanos' possession
As a result of its more esoteric properties, the Soul Stone proved the least utilized of all those in Thanos' possession despite the Titan displaying a mastery of its power. During the Battle of Titan, he only used the Soul Stone once to determine which Doctor Strange was the genuine article after the sorcerer had duplicated himself. 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.

After retrieving the Mind Stone, Thanos was stabbed by a vengeful Thor but retained enough strength to use all six stones, their powers now amplified to their full potential, to wipe out half of all life in the universe. This act briefly sent his consciousness back into the realm of the Soul Stone where he briefly communed with a reflection of Gamora's spirit in her child form, telling the only daughter he had ever loved that he had achieved his goal but solemnly admitting that fulfilling his destiny had cost him everything.

Capabilities
As one of the six Infinity Stones, the Soul Stone ranks as one of the most powerful artifacts in all creation. However, as it represents and has absolute control over the fabric of the Soul, resulting in its more esoteric and enigmatic powers compared to the other Stones, said to have power over the cycle of life and death, the Soul Stone is often considered the most dangerous by some, and holds a special standing even among the other Stones.

While the other Infinity Stones all possess a degree of sentience to a certain extent, the Soul Stone is unique in that it can seem to operate independently of a host, with a modicum of intelligence and purpose not exhibited by any of the other Stones. Uniquely, in its shrine on Vormir, the Stone purposefully imposed a terrible price to test the worthiness of those who would try to claim it - only upon the sacrifice of the soul of a precious loved one to the Stone, would it finally present itself to a wielder.

Aside from placing a price upon its own retrieval, the Soul Stone possessed other capabilities that imply a level of wisdom not shared by the other Stones. It is capable of cursing others, turning them into a wraith-like creature and granting them immortality and instant knowledge in certain occasions, as shown after the Red Skull was banished to Vormir by the Space Stone, the Soul Stone was able to use its power to force the former HYDRA leader to become its eternal keeper and a guide to all who would try to claim it. The Stone's power cursed him to become an immortal, ethereal wraith, bestowing him with the power to gain perfect knowledge of all beings who journeyed to the Stone's altar, and the knowledge required to guide said beings to its retrieval. Notably, after he retrieved and inserted the Soul Stone into the Infinity Gauntlet, due to its far more unique properties, Thanos uses the Soul Stone the least out of all the other Stones, but still displays a level of mastery and familiarity over its abilities. He showed the power to detect and attack the souls of others with the Soul Stone, as when attacked by an army of magical clones of Doctor Strange, cast by the Images of Ikonn, Thanos swiftly used the Soul Stone to track the real one and then combined it with the Power Stone, instantly destroying the false Images while spiritually attacking the real Strange, briefly pushing his Astral Form out of his body.

Interestingly, the Soul Stone has also apparently granted Thanos the uncanny ability to look into and obtain information from the souls of other beings, seemingly the same power that it granted the Red Skull. Thanos was able to recognise and call Tony Stark by name, despite having never personally encountered him before the Battle of Titan. Thanos could see and compliment the Avenger's own "curse", the knowledge of an impending and inevitable threat consuming his world and those he loved, noting the similarities to his own quest to save Titan and the Universe from inevitable destruction.

Perhaps its most perplexing feature, the Soul Stone also contains a mysterious pocket dimension, consisting entirely of an infinite expanse of shallow water underneath a dim orange sky, where the soul of the person sacrificed resides and can be communicated with. After Thanos forced himself to use the power of all six Infinity Stones to kill half of all life in the Universe despite having already been badly wounded by Stormbreaker, the sheer power needed to achieve such a feat nearly overwhelmed him in his weakened state, casting his soul briefly into the world of the Soul Stone.

Within, Thanos's soul encountered and could communicate with a reflection of the soul of his daughter Gamora, whom he had sacrificed to the Stone. While Thanos's soul was represented as his current, unarmored form, but without the Gauntlet or his near-fatal injury from Stormbreaker, Gamora's soul was represented as a child, standing under a Zen-Whoberi archway, implying that the sacrifice and the dimension's appearance changes into the form that the bearer most wanted to see.

After the completion of the Infinity Gauntlet and the Soul Stone's powers amplified to its full potential, alongside the other Stones, it allowed Thanos to wipe out half the universe with a simple snap of his fingers even while severely wounded by Thor. The Soul Stone's specific role in the genocide seemed to be erasing the souls of the victims.

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.