Deathless Ones

The Deathless Ones were soldiers who were once part of the Deathless Prophet's ancient order, where they have devoted their eternal lives to guarding the Divine Source. These soldiers, like the Prophet, also possessed superhuman capabilities as well as the immortality. The Deathless Ones speak in a low, almost metallic-like version of medieval Greek, similar to the Stormguard who spoke only in Japanese.

They were stronger, faster, and more durable than normal humans, including the Stormguard soldiers themselves encountered on Yamatai. Lara Croft encounters them in the ancient ruins of Kitezh. They were equipped with ancient, durable armour and weapons and technological resources extremely advanced for their time. Their ranks consisted of many soldiers, possibly numbering beyond several thousands.

Origin
The Deathless Ones were the Elite Guardians and Soldiers of Kitezh, with each of them receiving rigorous training and warfare expertise. They came from the Byzantine order of the Prophet, and were tasked with being the primary defence and security force of Kitezh.

Siege of Kitezh
However, during the Siege of Kitezh, the Prophet, in an act of desperation, turned thousands of his Elite Soldiers and Guardians into Immortal Warriors, also known as Athanatoi. These Guardians were almost successful at defending Kitezh, but when they saw their defeat approaching and knowing what would happen if the Source fell into Trinity's hands, they turned their weapons towards the ice on the mountains. This resulted in them bringing the ice from the mountains down upon the city, killing both the Mongols and the citizens of Kitezh. The Trinity envoy travelling with the Horde did manage to survive, only to be killed soon after when he tried to escape. Before he died, the envoy went into hiding deep within the city and began to study the Deathless Ones.

They continued to guard the ruins of Kitezh and the Divine Source for the next thousand years, killing anyone unlucky enough to cross their path in their eternal duty, including the descendants of the people who managed to escape Kitezh; the Remnants.

The Last Survivor
The envoy was trying to find a way out of the destroyed city and stockpiled weapons and supplies for this purpose. He took note that the Athanatoi patrolled the streets of Kitezh, killing off any survivors, even if it was one of their own people. He noticed something supernatural about the soldiers, who were now the Deathless Ones.

He killed a lone Athanatoi, only to his horror as the soldier became ash in an instant. But the survivor grew tired, he fell asleep near the ashes. However, after a short while, he woke up and saw that the ashes had disappeared and in their place was the corpse of the Athanatoi he had killed earlier. When the Deathless One reawoke, the envoy quickly killed him again. This time the soldier gave him a silent promise of return before turning into ash again. The survivor then fled the area and went into another hiding spot in Kitezh.

He also took note that they were not only learning from their previous mistakes, but their power and skills were also beginning to adapt each time they were reborn. The survivor came to the conclusion that no matter what he or anyone else tried, the Deathless Ones would never die. Calling them a mockery to all of his god's creations, he stated that now he understood the true nature of the Divine Source. He came to believe that whoever gazed at the Divine Source became immune to death at the cost of their humanity and morality, although this is not true.

The envoy realized that if a 'heretic' like the Deathless Prophet and his Deathless Army could cause such an atrocity such as the destruction of Kitezh, he feared Trinity would do numerous times worse, should they ever gain such power. He decided it would be best for the benefit of mankind if the Divine Source itself was left buried, entombed, and lost forever. He made plans for his final attempt to escape, but was wounded before he could reach safety. It is likely that the envoy was unsuccessful in his escape and perished in the ruins, leaving behind his notes as a warning should someone else enter the buried city.

The arrival of Lara Croft and Trinity
In 2015, Trinity once again journeyed to Siberia in search of Kitezh after stealing Lara Croft's research about the Divine Source, hoping to once again find it and use it to further their own ends. Lara herself arrived a few days afterwards.

Trinity footsoldiers first encountered the Deathless Ones while searching deep within the ruins, which resulted in their swift deaths. Using their signature Byzantine Fire, they burnt some of the squads alive, with Lara encountering their charred corpses a little after. She later encountered the ancient soldiers while trying to the leave the Flooded Archives after finding the Atlas, which showed the location of the main part of Kitezh where the Divine Source was hidden.

The Deathless immediately attacked her as well, in an attempt to stop her from getting away with the artifact. However, they failed and returned to the main ruins of Kitezh. Jacob later comments on the Deathless Ones after it is revealed he is the Deathless Prophet, believing that they brought the ice down upon the city in an attempt to keep the power of Source for themselves.

Final Battle
Knowing Trinity was back and in full force, the Deathless Ones started preparing for all out war, gathering their full might in the centre of Kitezh near the Chamber of Souls.

Lara Croft managed to arrive before Trinity however and was forced to fight some of the soldiers after her presence was discovered, continuing deeper into the city afterwards. Trinity blasted their way in from above and started sending in soldiers, who were promptly slaughtered by the Immortal Army despite their advanced weaponry. With the main bulk of the Deathless occupied with Trinity's forces, Lara was able to make her way up the main cathedral.

The Deathless then became more desperate, climbing the cathedral arguably faster than Lara all while still in their full battle armour. They also started turning their own trebuchets on the cathedral in an effort to stop both Trinity and Lara from getting any further, to no avail.

After killing all the Trinity soldiers, the Deathless quickly made their way to the Chamber in pursuit of Lara and Ana. Arriving just as the latter had shot Jacob and taken hold of the Divine Source, they cautiously closed in. After gaining Jacob's approval, Lara threw the Source to the ground, shattering it and releasing all the souls that had been stored inside. This caused the Deathless Ones to quickly disintegrate into nothing, ending their immortal lives permanently.

Powers & Skills

 * Divine Empowerment: The Prophet raised an army by training them & granting them their power through exposure to the Divine Source. As a result they have certain supernatural powers.
 * Immortality: Being empowered by the Divine Source, they are, in a sense, the "Living Dead", and have an unlimited lifespan, being able to survive centuries. They also cannot die, and if they are fatally wounded, they self-resurrect quickly, hence the reason for them being called "Deathless".
 * Regeneration: They can regenerate from non-fatal wounds. Jacob, a.k.a the Deathless Prophet himself, has regenerated from bullet wounds many times. Even without their armor, the Deathless Ones still take a very significant amount of damage to be actually killed, despite their inevitable revival shortly.
 * Self-Resurrection: Despite their regenerative abilities, they can still be killed if their necks or heads are harmed severely, which at this point they can "die". When a Deathless One is fatally wounded, they burn up and turn into ash instead of lying on the ground like a corpse, only to return completely healed later on in the same spot. This is proven by the envoy, who states that despite him killing the same man many times, he revived several times. This lead the envoy to conclude that no matter how he tries to kill the Deathless Ones, they will not die, calling them a mockery of all of his god's creations. One letter from the envoy says that a recently killed Deathless One had given a silent 'promise of return' to the scribe before turning into ash.
 * Superhuman Stamina: Since they are immortal, they have no need of air, sleep, water or food for survival. They also never tire at all.
 * Heightened Strength: The Deathless soldiers exhibit strength and muscle power much higher than a standard human, such as when one soldier was able to lift Lara Croft with a single hand and when multiple soldiers were able to quickly scale the cathedral walls despite wearing full armour.
 * Heightened Endurance: In battle, the Deathless soldiers wield heavy weaponry, wear full body armour, and fight for extended periods without tiring. They also have a heightened pain threshold.
 * Heightened Speed: They are shown to be able to move around areas more quickly than a normal human, despite them having incredibly heavy and thick armour, as well as large and heavy weaponry which they use with such ease and agility.
 * Warfare Expertise: The Deathless Ones are highly skilled warriors, fighters, swordsmen, marksmen, craftsmen, tacticians & trackers. They also show proficiency in coordinating their attacks.
 * Superior Combatants: They are remarkable swordsmen, shield-bearers, hand-to-hand combatants and archers. They are capable of using their ancient Byzantine Weaponry with exceptional proficiency. Despite Trinity's most elite soldiers possessing advanced combat skills and state of the art military technology and weaponry, they were still all wiped out by the Deathless Ones with their ancient Byzantine weapons and advanced combat skills.
 * Superior Coordination: Like most ancient military armies, they are calculated, coordinated and well led. They are very resourceful, as they have managed to build up, weaponize and equip themselves for centuries to come.
 * Byzantine Weaponry: They have been stocking up and creating technologically advanced equipment of their time for centuries. These include swords, shields, archery equipment, catapults and most notably, Greek Fire vessels.
 * Byzantine Armour: A type of thick, dark metal armor that they wear, with each having the Prophet's insignia on it. These armors, despite being incredibly ancient, are still much more durable than the body armors of the 21st century. These armors offer the Deathless Ones enhanced durability and much greater protection from even the most strongest of firearms. The armor is also equipped with various furs suitable for a winter environment.
 * Greek Fire: Their most powerful weapons are Greek Fires. These do not stop burning until they have destroyed everything they were set out to burn. Greek Fire cannot be stopped by water or sand. Greek Fire is bluish in color and completely smokeless, thus giving it the illusion of Holy Fire.

Trivia

 * While the Deathless Ones wear full body armour which hides almost all of their features, closer inspection reveals that you can see their eyes and the skin surrounding them, both of which look normal, underneath their chainmail. This coupled with the fact that Jacob also looks like a normal man, means the Deathless look like normal humans underneath their armour.
 * The Trinity envoy claimed to have seen "blue fire" within a Deathless Ones eyes, however this is most likely an exaggeration as they have completely normal eyes in-game.
 * They use Greek Fire in their clay grenades. This weapon was once used by the Eastern Romans due to its difficulty to be put out and because it burned even on water they used it in their navy. In the game Lara says it was made with Sulphur (which causes fire to turn blue and burn hotter) but historians still question how Greek Fire is made.
 * Their warfare march is based upon that of the Romans. When sneaking through the Kitezh ruins Lara witnesses the march, led by a Deathless One dressed like a general. The march includes standard-bearers, soldiers who carried their armies' flag, typically as inspiration to fight.
 * The Deathless Ones have unnaturally high resistance to fire. Unlike the Trinity soldiers, they don't react much when on fire and will continue to pursue and fight Lara. It is unknown if this is due either to their supernatural powers or their Elite Kitezh Armour.
 * Curiously when the Deathless General is speaking to his men, he says how they must not let the Divine Source fall into Trinity's hands and that they must obey the order of the Prophet, hinting that they still follow Jacob.
 * Even after the game is over and the Divine Source is destroyed, you can still find some Deathless Ones roaming the ruins of Kitezh. This is likely just for the sake of gameplay.
 * The Deathless actually speak modern Greek as opposed to the Byzantine variation. The exact reason for this is that it's not possible to accurately recreate ancient Greek as there is no exact way to know how it sounded.
 * Although there is a varient of Pontic Greek called Romeyka which acording to scholars is the living language closest to ancient/medieval Greek. The developers could have used this language for the Deathless Ones and it would not have been out of place, unlike modern Greek.