The Soul Gem is one of six of the Infinity Gems. The gems were once one sentient being and the Soul Gem itself remains sentient. The Soul Gem requires a host body and usually combines itself to the host and domains the host's will and spirit.
The Soul Gem can steal a person or beings soul leaving their body lifeless. It can also be used to peer into someone's soul or manipulate it. It can also be used to translate any language and make them instantly understandable to the bearer of the gem. It can also reveal the purity of the soul. Unless properly handled, the gem will steal a soul of any touching it.The Soul Gem can also be used to create forceful blasts known as Karmic Blasts which attack the spirit. Even without a host, the gem is capable of speaking, flying and shooting powerful blasts.The Soul Gem pairs well with the Mind Gem and together can achieve feats that alone they cannot.
Any soul taken is then transported inside the gem. This world is known as Soul World. It is nearly inescapable. Many lose their powers and abilities once inside.
In this place everyone inside knows every other person inside as if they have met before. It is a very peaceful and calm place.
Adam Warlock once became the 'ruler' of this world and said that he found a peace that he could never obtain in reality there. Only he has had the power to escape but chose not to. He has helped others escape, such as Drax and the Silver Surfer.
The Soul Gem has need of a host body and has had countless hosts throughout the eons of it's existence. It came into the possession of the High Evolutionary, but he never used the gem because he was unsure of it's power. He eventually gave the gem to Adam Warlock to help protect Counter Earth.
Adam Warlock possessed the gem for many years. Adam and the gem were close as though they were one. The gem once took in his spirit in order to save his life and provided him with a donor body and resurrected him, as well as Pip and Gamora. The gem slowly drained his soul into it, thus he cold not survive without it. It became his very life force. It began to control his subconscious and began taking souls without his actions.
The gem would come into possession of the Kree Empire and was used by the Supreme Intelligence to contain and control the warring blue and pink Kree minds located within his construct. The Silver Surfer removed the gem from him, which caused his mind to come apart.
The Soul Gem next fell into the possession of the Elders of the Universe. They attempted to use all the gems to kill Galactus. The gem fell into a black hole afterward that led to the Magick Realm. After this it came into the possession of the In-Betweener.
The In-Betweener maintained possession of the gem while he was imprisoned by his masters in the Nexus of All Realities. It was here that Thanos took the gem from him during the Thanos Quest. He collected all the gems and combined them for ultimate power. The events that followed were known as the Infinity Gauntlet. The space pirate Nebula managed to take the gauntlet from Thanos and temporarily wielded it's power. After Thanos' defeat, the gem was returned to Adam Warlock as a member of the Infinity Watch.
During the Infinity War, the gem and all the others came to the possession of the Magus, who wished godhood for himself as well. After his defeat, the gem was once again placed on the brow of Adam Warlock. The gem once tried to leave Adam for a new host, named Darklore, because it could not enslave Adam's soul to his will and remained a free spirit. Adam of course did not permit this to happen.
The gem is currently in the possession of Doctor Strange.