Depending on the tastes of the GM in question, an Antediluvian campaign is decidedly high fantasy: the Lord is a frequently angry entity, humans and nephilim are mighty but sinful, angels, giants, and monstrous races that drowned in the Flood are all present, and the lifespans of men were measured in hundreds, not decades, of years. Arcane magic was invented during this era and magic use was rife.
If running a campaign in Antediluvian times, the GM should modify a map of the Mesopotamian region, carving out his own kingdoms, dotting the land with the cities full of the wicked founded by the nomadic Cain, giving many of the lands to the nephilim, and populating the lands to the north, east, and south with monsters. If the GM wishes for a different, more epic geography, they should take a map of the super continent Pangea and divide it into vast empires and populate it with dinosaurs and ancient animal races; created by the Nephilim and the fallen angels.
For even more fantastic campaigns, assume that the Antediluvian world was highly technologically advanced; even more so than the modern world (9-10 TL).
All of the Judaic and the sub-Judaic monotheisms speak approvingly of what is usually referred to as "the flood". This flood killed every single human being on earth except for one family. The tale of the flood and Noah became a morality tale, and the moral of the story was justified genocide. The tale was not told in horrified tones of revulsion at Yahweh's monstrous crime of mass murder. On the contrary, Yahweh's actions were presented as being necessarily morally right; humans had been disobedient and therefore deserved to be nearly exterminated as a species.