The Divine Source is the main artefact that both Lara and the shady organization called Trinity hunt in Rise of the Tomb Raider. It has been discovered by the Undying Prophet more than a millennium ago and hidden away from the world by the descendants of his followers even after their exodus from the Byzantine Empire, their slaughter in Syria, and their flight to Siberia.
The Divine Source is a crystalline object, about 50 cm long, that emits a bright blue light. It seems to be not that heavy, as Ana and Lara both have no problems holding it. Additionally it is not very strong, as it shatters easily when Lara throws it on the floor. It seems that the crystal was only the vessel for the actual Source that resided within and escaped after the crystal was destroyed.
The Divine Source is said to grant immortal life to the possessor, a fact that has been interpreted as a legend and myth. But Jacob is the living proof of the powers of the Source. It is this power that makes it so irresistible to Trinity who hope to create an invincible army of faithful soldiers to bring about a new world order, while others have a more personal reason to find the Source: Lara wants to finish her father's research, proving that he was not a misguided fool as his colleagues had him cut out to be; whereas Ana mainly wants the Source to battle her fatal illness and does not shy away from using Trinity to that means.
There is nothing known about the origins of the Divine Source. Some believe it to be extraterrestrial, while others like Trinity and the Prophet claim it to be of godly origins.
Jacob found the Divine Source some time before 970 AD, before he came to Constantinople and started gathering followers. It seems to have had a profound impact on him and his teachings. He then took the Source with him to Syria where he was killed in a Trinity attack, but soon resurrected by the power of the Source. The same thing happened again when the Mongols attacked the city of Kizeth, despite the fact that the Source was lost within the city walls and was now jealously protected by the Deathless Jacob created with its power.
When Trinity is about to find the Source in the old city after they stole the Atlas from the Remnant, Lara follows them and after much fighting with the Deathless and Konstantin and his mercenaries, she finds Ana clutching the Source. In the hopes of healing her sickness she looks inside and drops it; Lara sizes this moment to destroy the Source and with it the Deathless and Jacob, as well as her hopes of presenting the world with proof of the supernatural, finally having realized that the world is not ready for the power of the Divine Source and not wanting to let it fall into the hands of Trinity.