Night Sentinel

The Night Sentinels, first mentioned in Doom (2016) and later encountered in Doom Eternal, were the military force of Argent D'Nur who acted as guardians of the City of Argent and of the Wraiths prior to their civilization's fall to the forces of Hell. They appeared as technologically advanced knights wearing powered suits of white armor and primarily attacked with sword- and spear-like weapons. According to the lord of the Fourth Age of Hell, the Doom Slayer "wore the crown" of the Night Sentinels, and thus may have been considered their leader.

Ancient history
According to passages from the Ligra Sultagenta, also known as the Book of Kings, the race of the Night Sentinels called themselves the Argenta. They arose on a world that was impaled by a giant impactor known as the Cosmic Spear, which unleashed the beings of immense power known as the Elemental Wraiths (also called the "Firstborn" by the Argenta), and began the evolution of life on their planet. They were driven by the hostile environment and the power of Wraithcall to take up arms and in time created a highly advanced warrior culture, defeating giant magical creatures known as the Ancestrals in order to secure their place in the world of Argent D'Nur.

They crowned a series of warrior kings, beginning with King Ormero the Father. All kings of the Argenta were required to be warriors and were expected to lead from the battlefield in times of war; those who could not were deemed unfit to rule.

The Order of the Deag was founded as a priesthood to tend to the Wraiths, which the people of Argent D'Nur both worshipped and feared. Through immersion in the Wraithcall, the people of Argent D'Nur mastered both magic and technology, and combined them. Together, these made the Night Sentinels, the core of the legions of the Argenta, a bulwark against the forces of Hell.

The Time of Grief
In a period known as the Time of Grief, during the rule of King Etrex, the Argenta first made contact with the Maykrs, who brought many gifts to Argent D'Nur and sought to ally with its people.

Diaspora
In time, the Argenta spread out from Argent D'Nur using the machines and magic of the Maykrs into what they called the Empyrean Void - evidently what is otherwise known as outer space, from its depiction in illustrations. They sought to spread the word of the Khan Maykr, which at that time they considered to be enlightened. In this process they made contact with many worlds and peoples previously unknown to them and made many alliances.

By the time of King Novik, the relationship between the Argenta and the Makyrs had grown strained. The Khan Makyr had secretly compelled the Order of the Deag to swear loyalty to her alone, and it is implied she used them to lead the elite of the Night Sentinels into a trap, possibly at Hell's request.

Fall of Argent D'Nur
A "lowly" Hell priest named Deag Grav was finally able to corrupt a single Night Sentinel by promising the return of the soldier's son, who had been lost in combat against Hell. In return, the Sentinel would lead the Hell priest and his forces to the Wraiths. Deag Grav used the power of the Crucible to destroy the Wraiths and forged the source of Argent energy, known as the Well, from their souls. This began a period known in Hell as the Ascension, and immediately led to the fall of Argent D'Nur in a massive attack which culminated in the Icon of Sin being unleashed.

As repayment for his betrayal, the Sentinel's son was indeed returned, but in demonic form.

Post-fall
It is unclear how many of the Night Sentinels survived the fall of their civilization, but several are shown fighting alongside the Doom Slayer in a page from the Book of Daeva apparently after or during its fall. Accordingly, upon freeing the Crucible from the control of the demons, the Doom Slayer also releases five of the Sentinels' souls from captivity. Before the Slayer is tethered away by Samuel Hayden, the apparent leader of these spirits salutes the Slayer. Four of these spirits later reappear during the Slayer's sojourn through the remains of Argent D'Nur and assist him by leading him through the ruins to the resting places of the Wraiths, and finally to the location of Olivia Pierce.

Four sarcophagi almost identical to that in which the Doom Slayer was entombed can be found in the Advanced Research Complex shortly after leaving Samuel Hayden's office. The lids of these sarcophagi also bear the Slayer's mark, but in white rather than red. This suggests that the final followers of the Doom Slayer were entombed in much the same way as he had been, but that they did not survive their entombment and instead became captives of Hell until later freed, while the UAC later found their sarcophagi, now empty.

Statues of two Night Sentinels are kept by Samuel Hayden in his office. These statues depict the Sentinels as having carved the mark of the Doom Slayer into their armor as a sign of their allegiance to him.

Locations
Several locations are associated with the Night Sentinels and the culture of Argent D'Nur:
 * Argent Breach, the long-abandoned tombs of fallen Night Sentinels.
 * Argent D'Nur, the remains of what was apparently their capitol city, now absorbed into Hell.
 * Empyrian, the destroyed floating remains of a temple to the Wraiths. Other damaged structures can be seen in the distance.
 * Sentinel Prime, a currently largely unknown location featured in pre-release concept art for Doom Eternal.

Multiplayer
The Templar series of multiplayer armor pieces are based on the Night Sentinels' armor.