This is the period before actual, recorded history, but about which we still have some information - even if most of it is legend. Humanity, by this point, has the run of the place, though they still have some scattered competition; fierce, though dwindling.
Most fantasy tropes and devices could find a home somewhere in here.
It can be subdivided into three rough eras:
25 kYA to 17 k YA
Following the ice age, in what would come to be a 3000 year interglacial period, a civilization of human origin would develop on what was left of the Atlantean sub-continent (now about 1/3 of its original size). The Ten Kingdoms, with their ringed capital and immaculate roads, are probably the Atlantis Plato wrote about. This culture, while far ahead of any contemporaries, was a feeble parody of the Titanic Age: a medieval super-power in the early Bronze Age, but unaware that their homeland was tectonically doomed and that history would doubt their very existence.
At about 15,000 BC the continent sank in its entirety for reasons entirely geologic.
12k- 17k YA
The period between the sinking of Atlantis and the start of knowable history. The term was coined by RE Howard, and this was the setting for Conan and Slain and related works.
The Ice Age was in its last advance, and the giants and dragons were an extinction-level threat to mankind. Formal study of magic largely disappeared with Atlantis, but humans found strength in numbers and heroics.
5k-12k YA
This is the time of Legends, when the heroes of mythology walked the Earth, when you could plausibly shake hands with Herakles or Gilgamesh. The Ice Age is over, magic is mostly legend. This period bleeds into Known History.