From the introduction:
It is a distant future, far from the memories of Old Earth. There was once a great empire of golden, undying gods. An Orokin empire. These immortals created great wonders… of which many would lead to their doom. They created the Sentients, terraformers of immense adaptability and similarly undying, like their creators. They were rulers of the Corpus; enterprising merchants and scientists, and the Grineer; robust and hardy, but little more than slaves. On its edges, the Empire kept the Technocyte infestation contained and held an iron grip over the youngest children of the empire, the Tenno. All rebelled, and all saw that the golden lords could bleed... and die.