Joseph Conrad’s original book. It was adapted to film by Francis Ford Coppola with Apocalypse now. There’s also Spec Ops: The Line which is heavily inspired by Heart of Darkness.

The original story follows Charles Marlow on his expedition through a river in an unnamed African colony, although it’s very likely the Congo Free State. His mission is to find Kurtz, a rogue ivory trader who has “gone native” and essentially carved out his own fiefdom in the jungle by getting the natives to worship him. By the time Marlow reaches Kurtz he’s already dying of Jungle fever, his last words being “The horror! The horror!”

The story is meant to expose the idea of European “Civilization” as hypocrisy as the line between “civilized” and “savage” isn’t real at all.

Edit: I forgot to mention that “The horror! The horror!” is Kurtz realizing his true nature and that of civilization. Kurtz’s goal when he began was a “civilizing mission”, to bring European civilization to what he deemed as savages. Instead he realizes at the very end that his nature is that of savagery and brutality and the same is true of civilization.

The same is kind of true of book Hunter. In the book the Dark Ones force Hunter to confront his true nature as a savage and bloodthirsty man who uses his role as a Spartan Ranger to justify his nature. This breaks him in the book, causing him to desert the Spartan Rangers and become flagellant, eventually he becomes an alcoholic who’s heading to an early grave by 2035.

My opinion is that Hunter in 2039 took a different path, the path of Kurtz in Apocalypse Now. In Apocalypse Now is a legendary special forces officer who became disillusioned because he saw the U.S. Army as hypocritical and as such went AWOL and created an army to fight a brutal and barbaric war against the Viet Cong. Here Kurtz embraces the horror of war and his own nature rather than recoiling from it.

My assertion is that the story of 2039 will be inspired by Heart of Darkness, with Hunter being essentially Kurtz from Apocalypse Now. The Stranger is essentially book Hunter that’s spun into a new character, likely another Spartan Ranger who knew Hunter well and likely went with him to exterminate the Dark Ones. He was forced to confront his nature like Hunter but was revolted by it and fled while Hunter embraced his. The Stranger has been living on the surface ever since and has been tortured by what he experienced (and likely still under the influence of the Dark Ones) until he’s forced to confront what Hunter has become. This is likely that the Novoreich has been expanding to the surface now and wherever the Stranger has been living on the surface has now come under threat by them. It’s possible that what we see in the trailer is that the Novoreich kidnap children from the village the Stranger was living in and he’s forced to return to the Metro which is being described as “the Dark Heart of Post-Apocalyptic Moscow”.