Gravely injured, and gravely tainted by the corruption of the world, as made manifest by the demon that attacks him. But as the game starts, he is resurrected. He is lead underground to destroy the power cell, but the corruption is too strong, and he cannot reach it. It still sits in the heart of the world, corrupting it and its denizens from within the shadow monsters, the tendrils attacking him. But the power cell did not get destroyed. The Drifter fought the titans, and defeated them. Either through combat or sabotage, the cell caused an explosion, which ravaged the earth and created a rift, summoning the titans into the world.

But not everyone welcomed this invention. This effect can be seen in the intro, as the bird-shaped thing in the air. It would bring immortality to the world, by uniting the souls of its inhabitants. The giant diamond shaped thing is this cell. THE ABHORRENT CELL STILL FESTERS DEEP IN THE CHAMBERS OF THIS WORLD HARNESSING A GREAT WELLSPRING, A PERFECT IMMORTAL CELL WAS CRAFTED TO BE IMBUED WITHIN ALL SENTIENT LIFEĪ NOBLE GOAL, THOUGH SUCH A POWER TERRIFIED OTHERS, AND BROUGHT RUIN AS ITS PURPOSE WAS TRANSMOGRIFIED

The greatest advancement is described in the library: The monoliths tell of a people that made many scientific advances, always driven by a lust for advancement. The backstory is shown in the opening cutscene and further elaborated by the monoliths, scattered across the world. This is partially thanks to some speculation with other people, partially thanks to the awesome efforts to decipher the monoliths, and partially my interpretation.

Hyper light drifter walkthrough