Jacob

Constantinople Era
Jacob was born in Syria & was a wise citizen of the Constantinople Era. One Day he discovered the "Divine Source", to which he was exposed to & gained his immunity to death, illness, age & injury. He used his power to heal others who were not supernatural beings. This combined with his wisdom rendered him as a "Messiah" sent by God himself & thus gave him the name "The Deathless Prophet".

This put him in a conflict with Trinity, who were searching for the "Divine Source". During a great battle, he was seemingly killed by Trinity forces & his people fled Syria and left towards the Siberian area. Many believed that the Prophet's body was no where in the battlefield.

Trinity chased him & his allies all the way to Siberia. Once there, they built the city Kitezh & created the Chamber of Souls, a vault used to seal away the "Divine Source". Once there, he gave "the deathless power" to all of his armies, which were thousands of them, all of them turned un-killable.

During a great battle between the Deathless armies vs. the Mongols & Trinity, Trinity was shocked to see the Prophet alive & with no visible wounds. During the battle, to Jacob's horror, the "Deathless Ones" managed to bring all the ice from the mountains down upon the city. Thousands of citizens & enemy soldiers were killed in this genocide. The Prophet managed to seal away the "Divine Source" in the Chamber of Souls, escaped Kitezh & fled towards the remaining survivors who were around the valleys, surrounding Kitezh. The Deathless ones then started patrolling the streets of Kitezh, wiping out any survivors, stocking up technologically advanced weaponry of their time & making it their eternal duty to guard the "Divine Source", by any means necessary. Kitezh would then go on in mythology to be nothing more than just an "Atlantis" of the Russian Mythology.

Rise of the Tomb Raider
Jacob first appears in Rise of the Tomb Raider as a prisoner in the Gulag. There he meets and befriends Lara Croft, who frees him from his cell in exchange for information regarding the Divine Source.

During their escape from relentless Trinity forces, Jacob saves Lara's life after she nearly drowns. Lara believes Jacob to be a good man, however, she acquires the feeling "he knows more than he's letting on."

Later, Jacob and Lara traverse through the old copper mines, however, they are separated and Lara is forced to make her way to the Remnant Valley alone where she encounters a hostile Sofia, who nearly kills Lara when she suspects she killed Jacob. However, Jacob arrived just in time to vouch for Lara, ordering his people that "no harm will come to her."

It is later revealed that Jacob is just another alias used by the Deathless Prophet, who has been alive for generations, living with the remnants in the valleys and mountains that surround the Lost City of Kitezh.

Death
Ultimately, when Trinity besieged the ruins of Kitezh one final time, Jacob arrived at the Chamber of Souls, only to be shot several times by Ana.

After Lara destroys the Divine Source, Jacob returns to his mortal self and tells Lara he's never met anyone as extraordinary as herself in all his time, and thanks her for making this difference before he peacefully disintegrates.

Powers & Abilities

 * "Divine" Powers: He was exposed to the "Divine Source" and therefore gained supernatural capabilities. His powers were much more advanced than the Deathless Soldiers, likely due to him being much more wiser than the latter.
 * Exceptional Physicality: These "Divine" powers have mede him much more stronger, mobile & resilient than an average human being.
 * Semi-Immortality: Due to his "Divine" powers, he has not aged ever since exposure to the Divine source. He also has no need for sustenance(i.e. air, food, drink, rest) etc......
 * Regenerative Healing Factor: Any wounds, amputations or infections will always be healed within a short timespan. When he was crushed by big boulders, he returned an hour or so later, with no visible injuries.
 * Self-Resurrection: Should he disintegrate or be beheaded, his death will not be permanent, as he will automatically resurrect within a short timespan & all injuries will be healed by his "healing factor".
 * Restoration / Healing: Via practice over is powers, he learned how to heal any mortal being within several seconds, just by using the "Divine" energies his body produces. He can even sue such an ability to restore deceased beings. However, beings who has been dead for some time will not resurrect.
 * Hand-to-Hand Combatant: Due to his centuries of combat experience, combined with his superior physical attributes makes him a formidable combatant & marksman.
 * Warfare Expert: Like most of his people, he too is a highly skilled guerrilla fighter, marksman, stealth tactics, craftsman & hand-to-hand combatant.

Trivia

 * After finishing the game, some villagers around the valley can be overheard talking about Jacob and the source, one saying "Jacob and the source are gone, what are we protecting now" meaning they all knew about Jacob, he hadn't tried to keep himself and his immortality a secret to his people.
 * When it is revealed that he is the Prophet, Jacob tells Lara that he can still "feel pain, that he is still human, just without the human end."
 * At the end of the game, when the player returns to the ruins of Kitezh, the Deathless Ones are still encountered instead of Trinity soldiers. Either this means that some power of the Divine Source is still active, or the game developers overlooked this so the player could fight the Deathless Ones at anytime. There is a slim chance that Jacob is still alive even if the former is true, as he had completely turned to dust.
 * Jacob's voice actor is Philip Anthony-Rodriguez, who also voiced for Fallout 4, which had come out at the same time as Rise of the Tomb Raider.