The armor of the Doom Slayer appear to have it's own story in Doom. Apart from being an amazing battle-suit, the Praetor suit is an important asset of the game.
The origins
One of the testaments mention an entity named "The wretch" that offered the slayer an impenetrable and unyielding armor after he defeated the titan. This seems like is the greatest achievement that the doom slayer has, as the titan's dead body is the battlefield for the cover image.
One hologram of the praetor suit and the slayer's sarcophagus is displayed in Olivia's Pierce private laboratory. Olivia studied the slayer and his history in an attempt to stop him from get in his way of opening an permanent portal between dimensions.
From the demon scripts we find out that after the doom slayer defeated the titan, the demon priests demolished The Blood Temple over him in a last try to stop him. Defeated, he was placed in a cursed sarcophagus. The suit was separated from the doom slayer and encased in stone to stop them to reunite; if he will ever be awoken again.
The reunion
The priests burned the slayer symbol upon the cursed sarcophagus to stop the man who inspired fear like no other in demons hearts, as a warning to everyone that the sarcophagus must never be open again, the slayer must never be freed.
The symbol of the slayer can be found everywhere in hell, the symbol that the slayer wears it on his helmet.
The only symbol that can scare a demon it's the perfect guide for Dr. Samuel Hayden to found the tomb.
By studying the helix stone, he is able to find the sarcophagus along with the encased suit and retrieve them to Mars.
The modified suit
So, if the armor was ancient, and it was just an usual armor, we may assume that Dr.Hayden make some modifications in the lab and all the robotic arms weren't just suppose to carve the suit out of the stone, but to upgrade it too. This would also explain the English words present on his helmet, as well how the AI Vega is able to connect to Doom's guy suit and activate certain upgrades as the story continues.
The Night's Sentinels Order
In another testament is mentioned that "he wore the crown of the night sentinels". Statutes of him and 2 night sentinels are exposed in Samuel Hayden's office. From how they are positioned right next to his desk, we may assume that Hayden saw him just as a solution to the demon invasion, as well as he admired his power. Different from the other night's sentinels, a human lost in hell, capable to absorb the strength of his fallen enemies, the doom slayer can't be compared with any other sentinel, as the intro to Doom 2016 says "They are rage, brutal, without mercy. But you. You will be worse. Rip and tear. Until it is done." This can refer to the night's sentinels as well as the demons.
During the story after he defeated 3 hell's guards, 5 sentinels appear showing him their respect, as one of their own.
Images taken:1, The rest are screenshots of my playthrough.
