Near the end of the Dawn Age, a new force entered Westeros from the east. To the east of Dorne, where the Stepstones now stand, there was a great land bridge that spanned the Narrow Sea and connected Westeros to Essos. This bridge allowed a new people to wash into the continent like a wave.
No one is sure of who exactly these people were but when they crossed, they crossed in force. These First Men settled in Dorne at first but slowly crept northward until they covered all of Westeros. As they were an invasive species, it was only a matter of time until the First Men came into conflict with the native population.
The First Men were an agrarian society and brought the knowledge of farming with them across the bridge. This allowed them to raise small forts and villages across Westeros. As land became more scarce, they began to chop down the extensive forests that covered the land including the weirwood groves. Even the sacred weirwood trees with carved faces fell to the axe. This was an act of war to the Children of the Forest and caused a conflict that lasted for generations. But along with farming, the First Men brought with them bronze weapons which made them a significant threat to the Children.
The warriors among the Children could do nothing but slow down the advance of the First Men. The greenseers employed all of the powers they could muster to bring down the First Men. Tales say that these wise men could call the beasts of Westeros to fight on the side of the Children. But no matter what they brought to the table, the First Men always found a way to beat them back. This was the breaking point for the Children of the Forest and forced them to make very desperate actions.
The legends of the First Men say that the greatest of the greenseers met at Moat Cailin to weave their dark magics. They said that they were the reason the arm of Dorne broke into pieces. The Children attempted to do the same with the Neck to separate the North from the South but something went horribly wrong and caused the land to turn into a swamp. Some maesters debate whether or not this was caused by the Children’s magic or natural events. Whatever was the reason for all this, the Children fought just as fiercely as the First Men.
Eventually, though, people always become tired of war and crave peace. The wisest of both groups met on the Isle of Faces in the Gods Eye lake to form the Pact. This gave dominion over the deep forests to the Children and made the First Men promise to never cut down another Weirwood tree. The Pact was sealed with carving faces into every weirwood tree on the isle, which is where it got its name. With this agreement, the Dawn Age of Westeros ended and what followed came to be called the Age of Heroes.