The Old ones are a peaceful reptilian spacefaring speceis which developed an interstellar civilisation which was at its peaks some 60 millions years ago, becoming practically immortal and able to undertake great works of psychic engineering. With a slow, cold blooded wisdom, they built a network of Warp Gates connecting the worlds of the galaxy, seeded on them new intelligent species and reshaped thousands of worlds according to their environmental and geographic criteria.
Upon meeting the Necrontyr upon their space journey, a war erupted between the two species, which was quickly over due to the Old Ones superiority. But later on, the Necrontyr allied themsleves with the C'Tan, a powerful non-corporeal species and managed to push the Old Ones to their limit and caused them to unleash powers they were unale to control, forcing them to retreat, their empire in ruin.
The Old Ones left behind species they had helped mature and others they had created but from that point on remained absent from the Milky Way.Among their children races were the Eldars, the Jokearo, the Rashan and K'nib, the Korks and possibly the Slann who might also be a descendant or subject species.
Source: Warhammer universe.