As best I can tell, sometime after the Beginning, the Deep Ones had a falling out an one of their number, Cthulhu, came to earth to claim it as his own. Unfortunately, the stars were not right, so Cthulhu sunk his city-tomb to wait out the eons. Around this time his cousin Dagon also came to earth and settled in the sea.
Some time after this, the Forerunners arrived, populating the galaxy from some unknown place of origin. They loved Earth above all their possessions, and when the Flood came along, made plans to allow the humans to inherit their vast knowledge. It was a good thought, since the Flood proved too much for the orderly Forerunners, who disappeared into their shield worlds and touched off the Halo rings, exterminating the Flood.
Though future investigations turned up nothing, in fact the Forerunners had discovered a way to break through the barriers of space and time, stumbling into another dimension in the distant future.
There, bereft of their technology, they set up a tribal existence so basic that the Imperium of Man, just arriving to explore the region, dismissed the newcomers as an indigenous species. In time the Forerunners, rediscovering their discarded technology, rose up to create a new empire, calling themselves the Tau and attributing their technological leap to their faith in “the Greater Good.” Unfortunately, the Forerunners/Tau had been followed by their Flood adversaries, who had fallen into the new reality at a different place and time. Mutating and reforming, the new Flood fleets descended upon the galaxy with new forms that were termed by the Imperium, “Tyranids.”
Meanwhile, back on Earth, the reseeding of the Human race went well and three principal races had formed: the Atlantians, the Lemurians, and the Picts. But at the start of the Hyborian Age a cataclysm shook the world and the first two empires collapsed, the survivors spreading out to form sundry new nations of Hyrkanians, Hyborians, Picts, and the tribes of the south. It was during this time that the Deeps Ones began to creep back into memory, for a legend grew that either in that age or the one previous, the followers of Bokrug the water lizard, the forgotten lords of Ib, had overthrown the city of Sarnath beside their acrid lake.
Lost cities and temples dedicated to grotesque gods were occasionally discovered in pools and forests or hidden in the distant horizons, and in the tiny realm of Khauran, a great toad-like monstrosity called Thaug was installed by the witch Salome as a voracious puppet-deity. Thaug was perhaps the most visible of these dark religions, for he met his end when an invading army of Zuagirs, under the command of a Cimmerian named Conan, stormed the temple on behalf of a deposed queen. In time, the Picts came to rule the known world under their brutish king, Gorm.
Further cataclysms brought an end to the Hyborian Age and reshaped the landscape. This most destructive event also had the curious side effect of spinning off multiple realities, one of which would come to play host to the Forerunners/Tau and the Flood/Tyranids.
As for Earth, the nations of the world realigned themselves and history progressed largely as we have always believed. In the north of the Americas, a great nation called Lomar rose up to rival that of the Inca and Aztec and Maya, but fell with its capital city, Olathoë, to a wild tribe called the Inutos, later “Esquimaux,” who left no trace of the proud civilization. By the 19th Century, the enduring tales of Cthulhu were dismissed as the ravings of madmen, though a sailor claimed to have stumbled upon the alien’s city-tomb and encountered the nightmarish beast in person. The claim remains unverified.
All materials are not mine, but are collected from the fictions of H.P. Lovecraft, Robert E. Howard, Games-Workshop Ltd, and Bungie Studios
