Jacob

Jacob was a Byzantine Prophet who used the Divine Source to create an immortal army.

Jacob and his followers fled from Trinity to protect the Divine Source, settling in the east founding the City of Kitezh.

Byzantine Era
Jacob was born in the late tenth century, in the Byzantium area. At some point 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 Byzantium. Jacob created a fake tomb to throw people off his trail, before his people headed east to what would one day be known as Siberia.

Jacob and his followers, seeking to protect the Divine Source, built a massive city, called Kitezh. WIthin the city, Jacob 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 army to make them invincible.

Trinity eventually tracked him down, bartering with Genghis Khan, the began to lay siege to the city.

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, as the Deathless ones didn't want to risk losing the Divine Source. Jacob and the other survivors fled towards the area surrounding Kitezh. The city and the source were lost, and many of Jacob's people were killed. Jacob and the others settled in the surrounding area becoming the Remnants of the Prophet.

One soldier of Trinity, who had been hunting Jacob since Byzantium and had even once claimed to have killed him, was given food and shelter, and allowed to leave once he had recovered, leaving him alive with the knowledge that he had failed.

Kitezh would then go on in mythology to be nothing more than just an "Atlantis" of the Russian Mythology, whilst the Remnants stood poised to guard the city as their eternal duty.

Rise of the Tomb Raider
Jacob is captured and interrogated by Trinity, who know he is the Remnant leader. Jacob refuse to give up any information on the Source or Kitezh itself to Trinity. He is summarily thrown into a cell in case Trinity find a use for him.

Some time later, Konstantin arrives with another prisoner, Lara Croft. Jacob tries to make small talk, however Lara is dismissive of him and immediately begins to break out. Jacob managed to convince her to break him out of the cell by telling her, "I know what you're all after." Lara relents and helps him escape.

During their escape Ana and Konstantin who sent their forces forces after them, including a gunship. Jacob saves Lara's life after she nearly drowns. He brings her to a cave to recover, when she wakes up he asks why should would risk her life for the Divine source. Lara tells him about her father and how she came to realise he was right after seeing something she thought was impossible. Jacob asked her to help them fight Trinity instead, which Lara agreed to but refused to give up her search for the source.

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 loses his immortality and tells Lara he's only met a few as extraordinary as herself in all his time, and thanks her for making this difference and allowing him to finally die, before he peacefully disintegrates.

Powers & Abilities

 * Divine Empowerment: He was exposed to the "Divine Source" and therefore gained supernatural capabilities. Unlike everyone else exposed to the Divine Source, Jacob retained his sense of self, becoming a charismatic and prosperous leader to all his people for hundreds of years.
 * Enhanced Condition: These "Divine" powers have mede him much stronger as well as more mobile & resilient than an average human being.
 * Immortality: Due to his "Divine" powers, he has not aged ever since exposure to the Divine source, appearing to be to be in his late thirties or early forties despite being almost a thousand years old. He doesn't require sustenance to survive either.
 * Regeneration: 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. He even healed from ballistic wounds without any aid. This is something Lara took notice off and it helped her to deduce that the fact that Jacob is The Deathless Prophet.
 * 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".
 * Superhuman Stamina: Jacob can exert himself for long periods of time, he also is also unaffected by cold, being able to swim through a freezing river to save Lara and suffer no ill effects.
 * 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 use 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 "I can feel pain, I can be hurt, I am human.....save for a human ending".
 * 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 (The same day in some territories).