Ultrakill is a retro style FPS game heavily inspired by classic games such as DOOM and Quake set in an alternate future in which WW1 went on for 200 years, fought largely by blood powered machines of various types. An unknown amount of time after the war finally ended, humanity died out for reasons that have not yet been revealed, leaving the many war machines without a source of fuel, leading them to delve into Hell to feed off the blood of damned human souls.
You play such a machine, named V1, fighting your way through the various layers of Hell in search of blood. As you do so, you encounter other enemies such as demons and angels, specifically the Archangel Gabriel who, despite only appearing in person 2 times so far in the game (it's still in early access. The final chapter is not fully released), is the narrative main character. As you explore Hell, the impact Gabriel has had on every layer becomes clear and, when you finally meet him, it sets in motion a series of changes within Gabriel that change the fate of Heaven and Hell forever.
Most of the story and details of the world are revealed through hidden levels and written passages you can find, along with terminal entries about enemies. It's possible to go through the game knowing none of the real story going on around you.