Mars Core

Mars Core is the seventh level of the campaign in Doom Eternal. It is one of the more visually impressive levels, taking place in low Mars orbit, ending at a partly Hellified site on the surface of Mars. The last Runes in the game are found on this level.

Description
With the help of Samuel Hayden, the final Hell Priest has been located on Sentinel Prime - a city from your past. The only known path to this accursed place is through the dimensional gateway hidden in the core of Mars, wherein the lost city of Hebeth resides. The journey there will take time you can't afford to lose - a dilemma which calls for a more... direct approach.

Overview
After rescuing Samuel Hayden at the ARC headquarters, the Doom Slayer learns from him that Deag Grav, the final Hell Priest that he needs to kill to stop the demonic invasion of Earth, is hiding on Sentinel Prime, a lost city from the Slayer's past. He also learns that the only way to get to Sentinel Prime is through a portal hidden in the lost city of Hebeth in the very core of Mars itself. Since getting to the core normally will take time that the Slayer and the people of Earth do not have, the Slayer hits upon a plan to use the BFG-10000, a giant energy cannon designed by Hayden himself as part of the moon's anti-demonic defense grid, to blast a hole through the surface of Mars in order to get to the core.

Upon fighting through the base, killing every demon in his way, and then getting to the giant energy cannon, VEGA overrides the safety locks on the BFG-10000 so that the Slayer can use it to blast the hole he needs to make in the planet itself, just before removing the cannon's power source, the BFG-9000, from its core and evacuating the Phobos base. From there, he fights more demons amid the destroyed UAC base on Mars until he reaches an escape pod, which he uses to launch himself right into the core of Mars where Hebeth and the portal to Sentinel Prime and the final Hell Priest await him.

Objectives

 * Shoot a hole in Mars
 * Get to the BFG-10000
 * Retrieve the power source
 * Evacuate the Phobos base
 * Reach the portal
 * Use the Ion Catapult
 * Get to the escape pod

Mission challenges

 * Big Ba-Da Boom - Kill 40 Demons with a very big gun.
 * Disarmament - Destroy 3 arm cannon Weak Points on Mancubi.
 * Lock and Key - Find the Slayer Key.

Quick Travel Points

 * Phobos
 * Phobos Teleport Station
 * BFG-10000 Gun Deck
 * BFG-10000 Ventilation
 * UAC
 * UAC Outpost LD
 * Meteor
 * Alpha Labs
 * Martian Chunks
 * UAC Transmission Tower
 * Delta Labs
 * Hell
 * Temple of Sin

Trivia

 * The Phobos facility is described in the lore as an expansion of, apparently, the original Phobos base in the very first DOOM game.
 * As in Doom (2016), Dr. Hayden hints to the Slayer where he can get a BFG-9000; in this case, in the cockpit of the BFG-10000.
 * The Alpha Labs and Delta Labs quick travel points are references to the Doom 3 levels of the same name, implying that these same facilities were left floating in space by the BFG-10000 blast.
 * The Lost City of Hebeth, now exposed to the surface after blasting Mars, has been subject to Hellification like much of Earth, and is even described as "Hell: Temple of Sin" in the Quick Travel Points, even though strictly speaking it is on Mars. It is possible that it was one of the dig sites from the Doom 3 games, considering the Mars City facilities on that part of the planet were blasted into space.