The Old Ones are a mythical and mysterious race or possibly a group of distinct species in the table-top wargame Warhammer 40,000. They played a pivotal role in the history of many races, including the Eldar, Jokaero, Humans, Orks and Necrons.
In their travels, the Old Ones seeded many worlds with life, and encouraged the development of indigenous animal and intelligent life on many more. Earth is believed to have been seeded in this way as the Old Ones probably first seeded it with primitive, single-celled lifeforms.
It should be noted that the Old Ones in the 40K universe may be connected to the Old Ones in the H.P. Lovecraft mythos. From The Encyclopedia Cthulhiana "Old Ones. Term often applied to the Great Old Ones or the Elder Things...the Antarctic aliens known as the Elder Things,...". Which created life on Earth (Terra) in the H.P. Lovecraft mythos.
However, the Necrontyr then discovered that a C'tan, a member of a race of enigmatic energy beings who had come into existence not long after the act of Creation itself, was living within their home star, feeding on the sun's radiant power. In a short time, the Necrontyr managed to create a technology based on "living metal" or Necrodermis, that allowed the C'tan to manifest itself physically by enclosing itself within a construct made from Necrodermis. The Necrontyr soon began to view their C'tan and its fellows as gods. With the help of the C'tan, the Necrontyr were able to turn the tide of their war against the Old Ones, largely by transferring their own consciousness into robotic bodies forged of Necrodermis like their C'tan "Star Gods." Unfortunately, the process of conversion dampened the Necrontyr's emotions and intellects, turning them into little more than mindless, eternally anguished slaves for the C'tan, who discovered that feeding on the life energies of other intelligent beings was far more "tasty" than the bland energy given off by stars. After their transformation, the Necrontyr became known as the Necrons and began cleansing the worlds held by the Old Ones and their allied species, feeding whole planetary populations to the foul appetites of their C'tan masters.
In the conflict that followed, called the War In Heaven by the Eldar's lorekeepers, the Old Ones created and modified many new races to help them combat the Necrontyr. The C'tan had no ability to comprehend or manipulate the Immaterium, and thus the Old Ones came to the conclusion that they would create a series of races attuned to the Immaterium, in order to use the vast repository of energy present there against the C'tan. Of these races, a few survive today, such as the Eldar, Jokaero, Hrud, the Krork (who, it is speculated, eventually devolved or mutated into the Orks and other Greenskins) and to a certain extent Humans.
One such Daemonic race, called the Enslavers, managed to discover a way to force entry into the material realm, and began to psychically enslave huge numbers of sentient beings. This new invasion dealt a fatal blow to the Old Ones (all of whom were killed or fled) and forced the C'tan and their Necron minions into hibernation on the Necron's so-called "tomb worlds" across the galaxy some 60 million years ago, as the C'tan (who fed on the energy produced by the sentience of other beings) had their food supplies stolen from them. It is hinted in recent publications that some of the Old Ones did indeed survive the War In Heaven.
In the fictional universe of Warhammer 40,000, the Slann were a reptilian or amphibian species who were servants of the Old Ones (although some sources state that they in fact are the Old Ones) not unlike the Lizardmen of Warhammer Fantasy. The Slann are reputed to have created some of the major races in the Warhammer 40,000 universe, the most notable of which are the Orks (who refer to them as the Brainboyz) and Eldar (who are the only race that keeps records from the age) who were created with their psychic abilities to take part in the war between the Necron and the Old Ones in which they were all but wiped out by the C'tan and the Enslaver plague. The Human race was a race for which basic evolution was allowed to continue without the guidance of the Old Ones.
There were five generations of Slann spawned by the Old Ones. The Slann of the first generation have all died off, many likely killed in the war between the Old Ones and the Necrontyr. This is very similar to the story of the Slann race in Warhammer Fantasy.
There is a theory that the Ork Gods, Gork and Mork, were two of the last Old Ones. Gork being the Ork God of strength and Mork being more cunning than his opponent, this also leads to the idea that the Eldar Gods Kaela Mensha Khaine and the Laughing God are the same beings as Gork and Mork. Kaela Mensha Khaine is a god of strength as is Gork and the Laughing God was more of a trickster than Khaine, much like Mork. Being the Old Ones they would have created races such as the Eldar and Orks. These two figures are believed to be the last two Old Ones to have survived.