Night Sentinel

The Night Sentinels, first mentioned in Doom (2016), were the military 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-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.

Pre-fall
Before the fall of Argent D'Nur, the Night Sentinels stood as a bulwark against the forces of Hell and acted as protectors of Argent D'Nur's mystical Wraiths, terrible beings of immense power that the people of their land evidently both worshipped and feared. Due to the power with which the Wraiths imbued the Night Sentinels, the forces of Hell were never able to stand in battle against them.

Fall of Argent D'Nur
A "lowly" Hell priest named Daeg 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. Daeg 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.

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. The five 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 those 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.
 * Empyrean, 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.