The Soul Stone was one of the six Infinity Stones, the remnant of a singularity that predated the universe, representing the element of soul. Kept on Vormir and under the protection of Red Skull since 1945. The only way that Red skull would give you the sould stone was if you'd sacrifise that person who you love most "A soul for a soul." The Soul Stone allows its wielder to attack souls in a variety of ways, while also trapping souls in a pocket realm known as the Soul World.