Here is an overview of the game, via Microsoft:
In DOOM Eternal, you step into the armor of the Slayer, an unstoppable and unyielding force with a singular mission—Raze Hell and save humanity. Battle demons in an epic single-player campaign that takes you to Earth, to Mars, through Hell, and across dimensions never seen before.
Your arsenal of powerful upgradeable guns, grenades and abilities makes you a force on the battlefield as you blast, stab, and shoot your way through demon hordes. Take what you need to survive – health, armor, and ammo—from your fallen enemies during the heat of battle.
The story doesn’t end when Doom Eternal’s credit roll. Continue seeking your vengeance in DOOM Eternal: The Ancient Gods – Part One, launching on October 20 on the Xbox Store. The legions of Hell have razed the heavens, threatening to expand across dimensions. Battle your way to the heart of the threat and decide the fate of the cosmos.