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, sacrificing his 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.

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.

Capabilities
The Soul Stone is known for being the most powerful out of all the Infinity Stones. This is due to the fact that the Soul Stone grants its wielder dominion over life and death, giving the wielder the divine right to decide who lives and who dies. It can also be used to see into a person's soul, with Thanos using it to catch the real Doctor Strange after he had duplicated himself due to his soul.

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.