There exists a space between realities that defies the mortal mind. The motives, means, and logic of that realm is one of the truly alien forces. There are several noteworthy incursions of eldritch forces in history, though most were forgotten as they drove the witnesses insane.
There are occasional remnants of eldritch forces; prophecies of dark gods, impossibly ancient cities, and cults that reach back from before humanity. These relics point to a much longer history of earth, including several alien empires and associated wars.