The Old Gods lived in the world of Urgrund that existed in the prior universe who could manufacture weapon beyond the comprehension of any other being. This realm was noted for being a paradise and known as the Third World though it had also been known by other names such as Asgard and Valhalla. At some point, their world became bathed in blood with a war being waged among their kind where they butchered one another. The Mageddon weapon was sent across the shining bridge to obliterate Joyous Home that was the star-city of the Old Gods. During this time, one among their number known as Gog claimed to had condemned the war and refused to pick a side. As a result, he was cast out and fell through the cracks between dimensions across numerous parallel universes before crashing on Earth. In their final moment, indescribable power was unleashed that proved to be fatal for the Old Gods as their home was split asunder.
This destructive act that shattered the Old God's home led to the formation of two new planets known as Apokolips and New Genesis. On these worlds arose a new race of deities known as the New Gods who believed that it was the forsaking of purpose that destroyed the Old Gods. After their destruction, knowledge of their kind was unknown among the New Gods though Metron's research managed to determine some of their history. This included the archaeo-technology such as the Mageddon that was named from the language of New Genesis. On Apokolips, deep underground resided a ruined city where the undead Dreggs who were the bodies of the Old Gods that roamed the area aimlessly.
The New 52
According to the Trickster Kaiyo following the Flashpoint, the Old Gods once lived as deities above those that would become the New Gods. In this time, these ancient titanic beings survived on the faith generated by the beings beneath them. Such individuals prayed for their survival amid the clashes between the Old Gods. This was until a young Uxas tired of their ways and held nothing but contempt for them unlike his brother Ixaya. In time, Uxas decided to plot against the Old Gods and he traveled to their home where he whispered words to turn each of them against one another. In addition, he acquired a weapon that sapped the energy of these godly beings and slayed each one where their combined power turned him into the being known as Darkseid. One of the dying Old Gods was petitioned by Ixaya who asked the deity to save his wife Avia. However, the Old God instead in his dying moments gave his cosmic power to Ixaya turning him into the Highfather. Thus, the Old Gods died and the New Gods were born to replace them.