While it certainly is not a story-first game, DOOM: Eternal has a surprisingly deep story that fleshes out the origins of the doom slayer, the demonic takeover of Earth and an ancient race that once battled the forces of Hell.

BEGINNING

By the time the doom slayer returns to Earth, 60% of its inhabitants have been slaughtered and harvested by demons

DOOM: Eternal begins 2 years after the events of DOOM (2016). During this time, Earth has been invaded by hell. The remains of humanity have formed into the Armored Response Coalition and are led by Samuel Hayden, the UAC chairman who had their mind placed into a robot and the closest thing to an ally that appeared in DOOM (2016). The doom slayer, who at the end of the last game was teleported to an unknown location by Samuel Hayden, returns to earth to save it from destruction inside of the “Fortress of Doom” a large spaceship created by an ancient race, inhabited by the slayer’s AI assistant VEGA.

Saving Earth

Once the financial capital of Earth, now New York City is being transformed into literal Hell on Earth

In order to save Earth you must kill three Hell Priests that are running the hellification and destruction of Earth. They work for the Khan Makyr, leader of a realm called Urdak, who leads the armies of demons in their conquests. The doom slayer travels to New York City, which has transformed into fields of lava and destroyed buildings that are infested with demons. He finds and beheads the first Hell Priest, leading to the Khan attempting to delay the doom slayer by teleporting the other two priests into hidden locations, forcing him to search for them.

The Night Sentinels

The doom slayer’s search for the Hell Priests takes him to Exaltia, the homeworld of an ancient race called the Sentinels. The Sentinels once protected the realm of Argent D’Nur from the armies of hell, until Khan Makyr revealed herself to them, who came with the promise to aid them in their war with hell. They continued their war, now aided by Khan Makyr who seemed like a God to them until they discovered that the energy that Khan Makyr aided them with, Argent Energy, was powered by the souls of fallen sentinels and that sentinels were being used as slave labor to build the towers where their souls are extracted to be used by the Khan Makyr or the legions of Hell. This information began a civil war in their society, between the Night Sentinels and those who enjoyed living under Khan Makyr even with the knowledge of what the source of her energy is.

The Night Sentinels, with the doom slayer among their ranks, became an unstoppable force and neared victory before the Khan Makyr looked to Hell for aid. The Betrayer, a member of the Night Sentinel who gave up access to the Night Sentinel’s power source to a Hell Priest and in return would have his son resurrected. However, he was betrayed and his son returned as a monstrous beast named the Icon of Sin. Without their power, the Night Sentinels collapsed and their realm was conquered by the Khan. While on the planet the slayer retrieves a celestial locator and is given a key and dagger after encountering the Betrayer on the planet.

Hunting the Second Hell Priest

Now with the celestial locator in hand the doom slayer tracks the second Hell Priest to a UAC base in the arctic. While here the slayer comes face to face with the Doom Hunter, a modified monster created specifically to deal with the doom slayer, but much like every other attempt Hell makes on the slayer it fails. The slayer successfully beheads the second Hell Priest, Deag Ranak, forcing the Khan Makyr to teleport the final Hell Priest to an unknown location to protect her plan for Earth.

Taking Back Earth 

The Marauder never makes an entrance without style

In order to strike a heavy blow to the Khan’s attempts to destroy the planet, the slayer heads to the epicenter of the invasion, Central Europe, to destroy a demon ‘gore nest’ to hurt the Khan Makyr's transformation of Earth. After destroying the gore nest the next target is the final Hell Priest whose whereabouts may be known by none other than Samuel Hayden.  

The doom slayer heads to an ARC base to locate Samuel Hayden. When the doom slayer arrives at the ARC base he is given a powerful artifact known as the Crucible and also finding a crippled Hayden, damaged after his attempt at a counterattack against the demons failed miserably. As the slayer finds Samuel, an unknown creature appears through a portal. This turns out to be a Marauder, a Night Sentinel that betrayed their kind and aligned with the Khan Makyr. The doom slayer throws Hayden through a portal that brings him to the Fortress of Doom and the slayer duels and eventually defeats the Marauder.

"YOU CAN’T BLOW A HOLE IN MARS"

Not even Mars' mantle can stop the doom slayer

With nothing being able to block the slayer’s path, even Mars, the slayer travels to Phobos in order to shoot a hole into Mars’ core to uncover Sentinel Prime, ancient sentinel city located in a different quantum plane that is reachable through Mars core which is considered impossible by him. The slayer not being deterred travels to a UAC base on Phobos. 

He reaches the base and uses a massive gun called the BFG-10000 to blow a hole into the surface of Mars before launching himself down there. Once landing he continues his journey to Hebeth, the lost city in the core of Mars, and would reach the teleporter to Sentinel Prime. 

Origins of the Doom Slayer

The doom slayer once fought side by side with these warriors to hold the line against Hell 

As the slayer explored Sentinel Prime, he began seeing flashbacks once entering the gladiator arena, which showed him once being the champion fighter in this arena. These flashbacks reveal the slayer to have been found by the Night Sentinels, badly wounded, before being brought into the arena where he defeated anyone put up against him even with his horrible wounds. He screamed about having to ‘rip and tear’ through hordes of demons, which caught the attention of the leader of the Sentinels at the time, the Khan Makyr.

Eventually the demonic invasion of the Night Sentinels homeworld had begun and now they realized the doom slayer may be their only hope at stopping them, thus beginning the war against the Night Sentinels and Hell. During this time a rogue priest granted the slayer superhuman abilities in order to create the perfect demon slayer. He would go on to fight with the Night Sentinels against the demons up until the Betrayer caused the Night Sentinels to lose the war. He continued his fight until the demons, who realized they could not defeat him, decided to trap him, where he would remain until being found by Samuel Hayden. 

Back in the present time, the Slayer finds the final Hell Priest, Deag Grav, in the same arena he once battled in. He battles a massive demon known as the Gladiator and defeats it after a long fight. He then turns to Deag, who reminds him it is against the Sentinel Law to murder on sacred ground, which doesn't matter to the slayer who continues to kill the deag and leave. 

Khan Makyr's Final Plan

A reunion a long time in the making 

After the slayer returns to the fortress, the Khan Makyr hacks into the fortress and puts it offline and reveals that since the Hell Priests could no longer continue Earth’s destruction then the Icon of Sin will do it. The slayer uses the crucible from the ARC base to restore power to the fortress and travels to Argent D’nur to reactivate his own crucible, an energy sword that he had used when fighting off the legions of Hell with the Night Sentinels. 

While hunting for his Crucible, more flashbacks reveal that a rogue Makyr known as Seraph granted the slayer with his superhuman abilities to aid him in his crusade against Hell. The slayer eventually retrieves his Crucible and also discovers that the Khan Makyr had created a deal with the Lord of Hell when she was still aiding the Night Sentinels. The Khan Makyr would provide worlds for Hell to invade and harvest the population to create argent energy and in return a portion of this energy would be given to the Khan's world.

Stopping the Khan Makyr

Once an all-powerful false God, the Khan Makyr had gone too far once she caught the wrath of the doom slayer

Traveling through Hell, the Slayer finds a portal to the Khan’s homeworld of Urdak. He attempts to stop a ceremony to bring the Icon of Sin to life again by interrupting the ceremony and stabbing the heart of it with the Betrayer's knife to stop it from returning. Stabbing it breaks the connection between the Icon and the Khan Makyr but the Icon still awakens. Now awakened and free of the Khan's control the Icon of Sin heads to Earth to begin its rampage.

The Khan reveals that her world survives on the argent energy created from Hell's invasion of other worlds, and Earth must be destroyed or Urdak will be destroyed. With her world's survival on the line she fights the slayer in a bid to stop him but fails. She is defeated by the slayer and her world becomes another source of souls to harvest for Hell.

Saving Earth from the Icon of Sin

The towering Titan was no match for such a force of sheer power that the doom slayer is

After defeating the Khan, the slayer returns to Earth to stop the Icon of Sin, the resurrected Son of the Betrayer and destroyer of Argenta and soon Earth. The slayer successfully slays the beast by stabbing into its head so forcefully that his Crucible breaks in its head. The doom slayer goes on to rid the world of its demonic invaders as he narrates the ending, stating that his fight is “eternal.”

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