The Old Ones were one of the first species to appear in the Milky Way. They were a cold-blooded, reptilian species with highly advanced technology. They built an interstellar empire that stretched across the galaxy, and were directly responsible for the creation of several prominent species in the galaxy. The Old Ones first created the Webway, which the Eldar would later perfect and expand.
The Old Ones eventually came into conflict with the Necrontyr, however the Old Ones easily overcame every effort the Necrontyr threw at them. Searching for a weapon to use against the Old Ones, the Necrontyr discovered the C'tan and built them bodies out of necrodermis. With the C'tan by their side, the Necrontyr engaged the Old Ones in war once more. This time, the war did not go in favor of the Old Ones -- especially once the Necrontyr adopted necrodermis for themselves and became the soulless Necrons.
To combat this new threat, the Old Ones crafted several psionically powerful species -- the Eldar, the Krorks, the Rashan, the K'nib, and others. Unfortunately, the powerful psychic energies of these species and their connection to the Immaterium allowed for the creation of the Chaos Gods. Although the Old Ones were victorious in the War in Heaven against the C'tan, the aftershocks of what their creations had wrought in the Warp proved too much. They either led to the extinction of the Old Ones, or the Old Ones simply fled the galaxy.
The Old Ones are derived from Warhammer 40,000.