Night Sentinels

The Night Sentinel is an order of knights that protected the Wraith and battled the Hell Legion eons ago. They are somehow connected to the Doom Slayer since their souls often act as his guide during his journey in hell. Their armor design is quite similar to the Praetor Suit, which raises a speculation that the Doom Marine was originally a member or a leader of the Night Sentinel and he now the last of his kinds. During the war with Hell, one of their member betrayed the order by leading the scheming demon priest Deag Grav to get close to the Wraith, which leaded to the creation of the Well. As the result, the order and their realm fell to the army of hell and the soul of all Night Sentinels had to wander hell for eternity. It is highly assumed that The Doom Slayer is the last of their kinds or even being the one that betrayed them in the first place. Giving the fact that their souls act as a guide for the Doom Slayer, it shreds some light on the Sentinels that either the Doom Slayer is not a traitor or the Sentinels had forgiven him for betraying them.