In the booksIn the A Song of Ice and Fire novels, the hammer of the waters is a Westerosi legend. The legend says that from Moat Cailin's Children's Tower the greenseers of the Children of the Forest called upon their nameless gods to send down the "hammer of the waters" to break the lands of Westeros in two in a desperate attempt to stop the onslaught of the First Men. The histories say that the crannogmen grew close to the children of the forest in the days when the greenseers tried to bring the hammer of the waters down upon the Neck.
Unlike the shattering of the Arm of Dorne, the attempt on the Neck was unsuccessful. The Children only succeeded in flooding the land, creating bogs and swamps. Why the Children's magic failed them is unknown. However, the cataclysm proved the strength of their power and may have proved instrumental in bringing the First Men to agree to the terms of the Pact that ended hostilities between the two races and marked 4,000 years of friendship between the First Men and the Children.