While there are many accounts of their conception, one legend states that they were formed out of the many threads of the primordial Chaos that predated the universe. It is said that before the imprisonment of Chaos, it released slivers of itself that became known as the Outer Gods in order to combat God whom then later conceived and formed numerous more that made the Great Old Ones.
Due to their advanced age, the Great Old Ones are older and more powerful than angels, demons, monsters, and even deities. And it is even said that should they be released unto creation again, they will plunge the universe into darkness and death, a feat that makes them so much of a threat that it forces the angels, demons, and monsters to form uneasy alliances against them.
These beings then began to spread throughout the uncharted dark corners of the cosmos, slowly poisoning it with their influence. However, it was not long before God took wind of their existence and tasked His archangels with finding and eradicating them. The archangels were mostly successful, but were unable to fully prevail in their mission. Many of the Old Ones were unable to be destroyed, and thus the archangels resorted to imprisoning them in either different dimensional planes of existence, other planets, or uninhabited places in space. The remaining Old Ones were able to escape their wrath by hiding.
During the epoch of the Earth's time, several Great Old Ones found the planet and decided to claim it as their own, however, each time they would be thwarted by their own kind as they were not too keen on sharing. This sparked many wars between the Old Ones which are revealed to be the cataclysmic events that wiped the dinosaurs from extinction. As of now, the Great Old Ones are either inactive via slumbering in the darkest corners of the Earth or space, or active by telepathically commanding the humans that follow them.
Unlike their parents, the Outer Gods, they are not necessarily malevolent even though they have the power to end the world on a whim, but never actively seek the destruction of humanity. This is perhaps it is due to their vast power and bizzare alien nature. To a Great Old One, humanity just is not worth the trouble of extermination, but that by no means forbids them from toying around with them. The Old Ones are said to be amoral, once again defining their embodiment of the cold and uncaring nature of the universe.
Ever since their arrival on Earth, not only have the Old Ones been in conflict with one another, but were also warring with the Earthly deities that resided upon the planet. The war eventually ended with the deities imprisoning the Great Old Ones so as they would no longer pose a threat to the planet and poison its inhabitants.