Divine Source

The Divine Source is an ancient artifact of mysterious origins. It was discovered by Jacob during the era of the Byzantine Empire. After gazing upon it, he finds himself to be healed from grievous injuries and to live periodically. This led him to become the Deathless Prophet and build a religion around the artifact.

The Remnants & the people of Kitezh believe that the Divine Source holds a fragment of the soul of the Abrahamic God, and believe that is the reason why the Divine Source is able to give immortality. The Divine Source removes and stores the soul of whomever gazes at it, thus making them immortal. Though all of the Prophet's followers, Trinity, the Remnants and Lara believed it to be Divine, only Jacob (The Prophet) knew that it was not truly divine.

Discovery by Jacob
Jacob found the artifact long ago in a location known only to him and upon gazing into it gained immortality as well as healing abilities. He then used his new found supernatural powers to perform "healing miracles" upon people with ailments or who were close to death's door. The people soon started to believe him to be a prophet of the Abrahamic God and the Divine Source to be of divine origin. Jacob himself however believed it not to be divine.

Prophet of Constantinople
Word spread about the Prophet of Constantinople and the Divine Source. Trinity soon gained word and were interested in the artifact. They ordered the Prophet to be executed and the Divine Source to be acquired for themselves.

Trinity and the Prophet's pilgrims soon battled for the artifact, with Trinity killing the Prophet in the process. But despite the Prophet's death, his followers managed to acquire his body and secure the Divine Source as they fled from Trinity. The Prophet's followers built him a giant tomb in Syria. But before they could complete their work, the Prophet was revived with all of his wounds regenerated.

Exodus from Syria and Building the City of Kitezh
The Prophet and his followers soon fled from Syria with Trinity, who had learned of his rebirth, hot on their tail. After a long journey, they arrived in Siberia and stopped near a massive natural rock formation, which they took as a sign of their god. They soon settled around the valleys and even built a temple near the rock formation in order to honor the scout who discovered it.

The Prophet soon recruited the best architects, philosophers, warriors, scientists, artists, builders and craftsmen of the Byzantine Era to built the city of Kitezh. After Kitezh was built, the Divine Source was hidden and locked away in the Chamber of Souls, which is a large vault in the middle of the city. The Atlas was crafted to show its location.

Mongolian Invasion and the Destruction of Kitezh
During the Mongol attack on Kitezh, Jacob in a turn of desperation, used the Divine Source to give immortality and superhuman physical abilities to thousands of his Elite Guardians and they became known as the Athanatoi, the Deathless Ones who, alongside the people of Kitezh, fought and battled the Mongolian Invaders. When the Deathless Ones saw that they were losing, they took a very catastrophic action. The Deathless Ones pointed their weapons at the large icy mountains and fired, bringing tons of ice down on the city. This act not only destroyed the Mongolian Invaders, but also buried Kitezh under a glacier to secure the Divine Source.

The Prophet soon escaped Kitezh and rendezvous with the citizens of Kitezh that lived outside the city. These people start calling themselves The Remnants, who believe that guarding the Divine Source is a duty that they must pass down to their descendants. Using their Byzantine heritage, they developed tactics, weapon and combat skills which helped them survive for centuries largely undisturbed by outsiders.

Lara's search for Immortality
Richard Croft's search for the Divine Source led to his death, and later to the the events of Rise of the Tomb Raider, where both Lara Croft and Trinity were after the Divine Source. Lara continuously aided the Remnants in order to earn their trust and find out more about the Divine Source, though they still kept it a secret from her. Though Lara started becoming skeptical about the Divine Source's existence, after seeing Jacob miraculously heal Jonah, finally came to realize the true power of the Divine Source and that it was not actually Divine, just an extremely powerful biblical artifact. Upon realizing this she immediately demanded to be let into the Path of the Deathless, showing more determination to acquire the Divine Source before Trinity does.

The Truth about the Divine Source
Upon entering the ruins of the entombed city of Kitezh, she encountered the notes of an ancient Trinity soldier who was entombed in here after the Deathless Ones buried the city beneath the ice. He had left his notes as a warning should anyone ever follow his path. Through the notes she realized the true danger of the Divine Source's power, even convincing herself that Jacob was right and that the Divine Source is not meant for the world. This made her even more determined to stop Trinity, because she now realized and feared the atrocity they will commit with this power.

Destruction of the Divine Source
Despite the long battle between Trinity's army, the Deathless Army and Lara with the Remnants, Trinity's hunt for the Divine Source was a success, with Ana making it to the artifact first. After Trinity enforcer informs Ana that nearly all of their soldiers are killed off by the Deathless Ones, he is killed by Jacob. Ana retaliates by shooting Jacob a few times. Lara arrives, after killing off Konstantin, and holds Ana at gunpoint as Jacob and the Deathless Ones slowly approach them. Ana tries to use the artifact but Lara snatched it from her hands when the artifact's power weakened her. As she held the artifact and saw the Deathless Ones slowly closing in, Jacob gave her his blessing and permission to destroy it. By smashing it, she released all the souls stored in it. This in turn turned Jacob and the Deathless Ones mortal again. Though the Deathless Ones immediately and violently turned to ash, Jacob was disintegrating slowly and thanked Lara for giving him a chance to move on to the Afterlife as he peacefully succumbed to his extremely advanced age, blessing her for her courage, strength, skills and conviction.

Trinity's Attempts Salvage the Divine Source
After the Divine Source was destroyed, Trinity attempted to try and find another way to harness the power of the Divine Source, which they linked to the isle of Yamatai and the Sun Queen Himiko, who they believed still resided trapped in the body of Samantha Nishimura, they purchased the facility that Sam was being kept, and began to experiment on her. This caused Himiko to emerge and take control of Sam's body. Now free, Himiko escaped the facility to get back to Yamatai in order to ascend the throne once more, rampaging across Europe in the process.

Powers

 * Soul Absorption: It has the ability to absorb the soul of anyone who gazes at it. However destroying it will release all the stored souls and in return, cause those who used the Source to disintergrate into dust.
 * Immortality Bestowal: It can bestow immortality upon those who are exposed to it. This is due to the fact that as long as the Deathless One's souls are contained in the Divine Source, they cannot die by any means.

Trivia

 * Jacob is apparently the only person known to have retained his humanity after gazing upon the Divine Source. However, the Deathless are not completely devoid of emotion or individuality as evidenced through an old Trinity agents notes. Furthermore, there is no mentioning of the Divine Source stripping people of humanity beyond Jacobs own assumption.
 * Adding to this, what humanity means differs from person to person and unless the Divine Source is alive and has an agenda of its own it does not make sense for it to have a view on what humanity as concept/ideal is, nor does it make sense for an entire army to be stripped of it, yet one person isn't.
 * The Divine Source may be related to the "Ancient Astronauts" pseudoscientific concept. The Mysterious Figurine relic found in the Path of the Deathless resembles a humanoid in a space suit. Lara does not articulate this resemblance in her commentary on the relic, saying only "This is... particularly odd. It has elements of Scythian design... but... it looks like an... no. Not possible." However, the relic appears to have been made by an older group of human inhabitants who occupied the Kitezh valley in prehistoric times, centuries before the Prophet and his followers arrived there with the Divine Source.
 * To add to this fact, it may very well be possible that the Divine Source may have its origins from outside of Earth. The unique astronomical symbols on the Mysterious Figurine coupled with the strange, almost alien like, markings on the Divine Source could point to a possible extraterrestrial source for the Divine Source. Also, the Divine Source heavily resembles a fragment of a meteorite based on its unique appearance. This theory, if confirmed to be true, could explain the Divine Source's unique aesthetics as well as its strange and seemingly supernatural powers since its "magic" could simply be very advanced technology that was interpreted as supernatural or divine to the primitive minds of humans due to the fact that humanity has not progressed enough to reach that kind of level of technology.
 * There seems to be some sort of connection between Himiko and the Divine Source as Trinity believed that drawing Himiko out would allow them to acquire it.
 * A possible point of connection is between Himiko and the Divine Source is the fact that both share the same blue light which could mean that while Himiko herself never possessed the Divine Source she encountered something like it. However Himiko came across her powers though the effects were clearly very different from what the Divine Source could do as Himiko would need to transfer her soul into a new body as the ones she would possess would eventually age and decay.
 * Another point to keep in mind is that while the body Himiko's possessed would age and decay she would continue to live on in them meaning that whatever gave her this power granted her eternal life but not youth as the Divine Source did for Jacob.