Soul Stone

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, sacrificing his daughter Gamora to obtain it.

Creation
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.

Guarded by the Stonekeeper
The Soul Stone eventually found its way to Vormir, where Johann Schmidt acted as its Stonekeeper for over seventy years. In 2018, he was visited by Thanos, with his daughter Gamora. The Stonekeeper explained that Thanos had to sacrifice someone he loved in order to gain the stone, causing Thanos to tearfully push Gamora off of the cliff, obtaining the stone for his Infinity Gauntlet.

Capabillities
As one of the six Infinity Stones, the Soul Stone ranks as the most powerful artifact 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, the Soul Stone was enhanced to the height of its powers, with the Stone's power over the soul becoming so potent that, combined with the other Stones, it granted Thanos the power to erase the souls of others from existence on a universal scale at whim. Even with Thanos grievously wounded by an all-out assault from Thor wielding Stormbreaker, he still proved able to use all six Stones to wipe out half the universe with a simple snap of his fingers, with the Soul Stone destroying the souls of the victims. However, doing such a feat at the weakened state he was at the time took a great toll on Thanos and the Gauntlet, with the sheer energy required for such a feat scorching the Gauntlet and his left arm.