The player has access to various firearms, such as the Combat Shotgun, Super Shotgun, Heavy Cannon, Rocket Launcher, Plasma Rifle, Chaingun, BFG 9000, and Ballista. Melee weapons such as a chainsaw, the "Crucible Blade" energy sword and a retractable arm-blade called the "Doomblade" can also be used.
The arm-blade provides the opportunity for a larger variety of quick and violent "glory kill" executions, which drop extra health. The Super Shotgun is now equipped with the "Meat Hook", a ranged accessory which grabs on to enemies and slingshots the player towards them, functioning as a grappling hook, useful both in combat scenarios and environmental navigation.
The Doom Slayer's armor now includes a shoulder-mounted Equipment Launcher with the ability to lob grenades and ice bombs. Additional to it is the Flame Belch, a flame thrower which sets enemies on fire with it to drop armor pickups.
Finally, killing enemies with the chainsaw will drop ammunition. New movement mechanics such as wall-climbing, dash moves, and horizontal bars to swing from have also been introduced.
Like its predecessor, Doom Eternal invites the player to develop simple tactics. As the game gets harder, the player needs to use the advantages of fight arenas, and their bonuses, in more strategic ways. They also need to decide which enemy has to be killed first and how to move on the battlefield.
Creative director Hugo Martin has stated there would be twice as many demon types as in the 2016 reboot. There are new types of enemies, such as the Marauder and Doom Hunter, while others, such as the Pain Elemental, Arachnotron, and Archvile, have been reintroduced from previous Doom entries.
A new system called "Destructible Demons" is featured, in which enemies' bodies become progressively destroyed and deteriorated in combat as they suffer damage. Through this system, certain parts of a demon's anatomy can be destroyed to eliminate their ability to use their stronger attacks.
There is also a new lives system. During gameplay, players can collect 1-up items (which look like green helmets) scattered around the environment. When the Doom Slayer perishes in combat, he will respawn where he died if he has spare lives, instead of restarting at the nearest checkpoint.
The game features several asymmetric multiplayer modes, including "Battlemode". This game type is a 2v1 PvP multi-round match where two player-controlled demons take on one fully loaded Slayer.
There are five playable demons at launch, with more to be included in free updates post-launch. The five initially available demons are the Marauder, Mancubus, Pain Elemental, Revenant, and Arch Vile. In addition to regular attacks, each demon has a "summoning wheel", which contains a selection of four additional abilities.
An additional mode, called "Invasion", is also in the works, and is expected to be released as a free post-launch update. In this mode, players will be able to join others' single-player campaigns, fighting against them as demons. This mode can be turned off by players who wish to play the game solo.
Also new to the game is the hub area, called "Fortress of Doom", which players can visit between missions, containing a number of rooms with upgrades and gear locked behind doors. Players can also unlock a bonus weapon, the Unmaykr, after clearing six challenging Slayer Gates.
Eight months after the events on Mars, Earth has been overrun by demonic forces, wiping out 60% of the planet's population, under the now-corrupted Union Aerospace Corporation.
What remains of humanity has either fled Earth or have joined the Armored Response Coalition, a resistance movement formed to stop the invasion, which has gone into hiding after suffering heavy losses. The Doom Slayer, having previously been betrayed and teleported away by Dr. Samuel Hayden, returns with a satellite fortress controlled by the AI VEGA to quell the demonic invasion by killing the Hell Priests: Deags Nilox, Ranak, and Grav.
The priests serve an angelic being known as the Khan Maykr who seeks to sacrifice mankind. The Slayer teleports to a destroyed city and kills Deag Nilox, but the Khan Maykr transports the two remaining priests to unknown locations, forcing the Slayer to continue searching.
After retrieving a celestial locator from the Sentinel world of Exultia, the Slayer travels to Hell to retrieve a power source from an exiled Sentinel known as the Betrayer, who gives him the power source and a special dagger.
VEGA directs the Slayer to Deag Ranak's citadel in the Arctic, where the Slayer kills him. In response, the Khan Maykr moves Deag Grav to a hidden location and accelerates the invasion of Earth.
With no leads on finding the last Hell Priest, VEGA suggests finding Dr. Hayden, who knows his location. The Slayer goes to an ARC compound where he retrieves Hayden's damaged robot chassis and the demonic crucible.