Sentinel Prime is the city where the ancient Arena is located and is considered a holy place for the people of Argent D'Nur.
Constructed in the earliest days of their history, this coliseum served as a proving ground for trial-by-combat. Historical records indicate that the Argenta did not jail their own kind: rather, prisoners and criminals were granted the right to earn their chance at regaining honor through victory in the arena. Those who succeeded were given a place to fight in the front lines of the Sentinel army, to die in service to Argent D'Nur. This traditional use of the arena has subsided since the assimilation with the Dark Realm. The corrupted Priests now allow more violent exchanges, with armed captives pitted against demons for sport. The Priests of the Order Deag continue to hold undisputed power on Argent D'Nur.
Unlike most other sentinel cities visited by the Doom Slayer, Sentinel Prime is actually inhabited to some extent, as the Doom Slayer meets several Night Sentinels at the heart of the city. The population most likely lives in oppression and constant fear under their overlords, and appear to revere the Doom Slayer as a leader of the city rather than the Hell Priest Deag Grav - as the former was supposedly a sovereign there at some point. It is in Sentinel Prime that the Doom Slayer fights the great demonic champion of the arena - The Gladiator, and kills Deag Grav. According to Sentinel customs, spilling the blood of the Argenta-ascended class will result in the transgressor's banishment. Asylum amongst the Argenta will be denied to any who do not adhere to the rules of engagement. Thus the Doom Slayer was banished from the city.
Sentinel Prime appears to have a cascadian landscape alike most other Sentinel cities, and appears to be within a subarctic temperate region, dominated by vast mountains and conifer forests. A single moon is visible in the sky, with a reddish surface colour not unlike Mars. At the heart of the city lies the gladitorial arena, where the champions of yore would fight mighty battles, and where condemned criminals were offered a chance for pardon though trial by combat. It was here that the Doom Slayer once proved himself worthy of living amongst the Sentinels long before the city fell under Deag Grav's control.
- The name "Sentinel Prime" is a possible reference to the TransFormers character of the same name.