"The Children of the Forest, the First Men, and the Andals" is a featurette and part of "History & Lore" section of the Complete Guide to Westeros, a special feature in the Blu-ray of Season 1 of Game of Thrones. It is narrated by Isaac Hempstead-Wright as Bran Stark.
In the Dawn Age, Westeros was inhabited only by the Children of the Forest. Though little is known about them in the present, they were said to be small of stature, dark, and beautiful and no taller than human children when grown to adulthood. They lived in the depth of forests, as well as crannogs and villages, hunted with weirwood bows and armed themselves with blades of obsidian. Their wise men were the Greenseers, who possessed powerful magic and worshipped the nameless, faceless gods of forest, stream, and stone. According to legend it was they who carved faces in the trunks of weirwood trees.
Twelve thousand years ago, however, the First Men came from the eastern continent, crossing the Arm of Dorne. Riding horses and wielding weapons of bronze, the First Men cut down forests and weirwoods and waged centuries-long war against the Children until, eventually, the two sides sought peace. They met on an island in the middle of the Gods Eye lake, where they forged "the Pact". Men would be granted dominion over coastlands, mountains, plains, and bogs, while the deep forest would forever belong to the Children and no weirwood would be cut down. To signal the Pact, the Children carved a face on every weirwood tree on the island, which came to be known as the Isle of Faces.
However, the Pact couldn't withstand the invasion of the Andals, a race of tall, fair-haired warriors. They attacked with fire and weapons of steel and slaughtered the Children wherever they found them. They also burned the weirwood groves as they spread their own religion. Centuries of war raged on, and the Andals conquered every kingdom in Westeros except for the Kingdom of the North.
The Kings of Winter were able to resist the invasion and so the descendants of the First Men dwell in the North to this day, still worshipping the Old Gods. As for the Children of the Forest, the few survivors were said to have fled far north and not seen again. Most assume they are long dead while some believe they never even existed. They now live only in song and legend, and in the faces of the weirwood trees.