The Night Sentinels were the guardians of the Wraiths and acted as both their holy servants and a bridle for their awesome power. These guardians were imbued with Wraith energy and proved fierce opponents of Hell's legions and, according to the Ungmar Codex, almost triumphed over Hell but were betrayed by a member of their order. The unnamed Betrayer struck a bargain with a lowly Hell priest, Deag Grav, for the return of his son who had been lost in battle, and he led the priest and his cabal to the source of the Night Sentinels' power, the Elemental Wraiths. Daeg Grav cursed the Wraiths while they slept, and the Hell priests stole their energy for their own devices. Argent D'Nur was defeated, and their realm absorbed by Hell, along with the Argent Energy that led to the creation of The Well.
The Sentinels are somehow connected to the Doom Slayer, as their souls often act as his guide during his journey through Hell. While little is known for certain about the Night Sentinels, some have speculated that:
- The Doom Slayer was originally a member or perhaps leader of the order and is now the last of their kind. This is supported by the first part of the Slayer's Testament, which describes the Doom Slayer as wearing the "crown of the Night Sentinels"
- Others have suggested that they guide the Doom Slayer out of either respect for releasing the Wraith's souls, or out of having a mutual enemy in Hell's legions.
- Stone tablets and other "artifacts" in-game commonly depict the Doom Slayer and Night Sentinels fighting together, indicating that they have met before. The Doom Slayer's armor is very different; however, and he may simply have aided them in fending off the demon hordes.
- There is also the possibility that the Doom Slayer is the previously mentioned Betrayer, as in the documents mentioning the Betrayer speak of how, while the Betrayer's son was returned to life, he became the Icon of Sin. The Betrayer then turned on the armies of Doom, and that is the end of explicit mentions to him. Combined with how it has been stated that Hell had 'wronged' the Doom Slayer, it is possible to connect the two as one and the same. If that is the case, than the Night Sentinels may help the Doom Slayer because they see his rampant crusade against the armies of Hell as a sort of repentance for his previous betrayal of them.
Either way, the souls of the Night Sentinels appear to the Doom Slayer at various points throughout Hell to point the way or stand in solidarity with him.
- In Multiplayer, Players can use the Templar Armor, the wearable equivalent of the Night Sentinel armor.
- The Armor that the Night Sentinels use is visually inspired by 15th Century Gothic Plate Armor, used by the Holy Roman Empire.
- Their Helmets are also inspired by the Great Helm, which was used by 13th - 16th Century European Knights.