Dawn Age

The Dawn Age, the Dawn of Days, or the dawn centuries, was a period of indeterminate length during which the First Men who arrived from Essos colonized Westeros.
The oldest known inhabitants of Westeros were the mysterious and diminutive children of the forest, gifted with many supernatural powers, who for time uncounted lived alone throughout the continent of Westeros, except perhaps for the giants. The children worshiped the nameless gods of forest, stream and stone, and lived off the land. It is unknown how long the children lived alone in Westeros, nor where they themselves came from. The oldest records available only relate to the arrival of the First Men to the continent, from Essos, about 12,000 years ago.
However, the oldest runic records of the First Men, and records from maesters that claim to have lived among the children of the forest, confirm men have lived at the mouth of the Honeywine - where Oldtown stands - since the Dawn Age. Maester Jellicoe suggested the settlement at Whispering Sound began as a trading post where ships from Valyria, Old Ghis, and the Summer Isles could resupply and trade with so-called "elder races".
The First Men crossed into what in time would be Dorne from the eastern continent, using the land bridge that connected the continents (the Arm of Dorne). They came bearing weapons of bronze and shields of boiled leather, and brought the first horses and their own gods. They swept through the lands of the children, cutting down and burning their sacred weirwoods to clear land for themselves. Outraged by the destruction of their homeland, the children took up arms.
It is believed that the wars between the children and the First Men went on for two thousand years, and though the children fought fiercely the war went in the favour of the more numerous, larger, stronger, and more technologically advanced First Men. It is said that during that time the children used powerful magic in a futile attempt to end the invasion, breaking the Arm of Dorne. Eventually wise men of both races forged the Pact at the Isle of Faces, marking the end of the Dawn Age and the beginning of the Age of Heroes.