The Neck is a swamp and marsh-filled region of Westeros, located where the waters of the Bite (an inlet of the Shivering Sea) and Blazewater Bay (an inlet of the Sunset Sea) draw relatively close to one another, making it the narrowest part of the continent.
The northern end of the Neck is controlled by House Stark via the formidable (but usually unmanned) castle of Moat Cailin and their vassals, House Reed of Greywater Watch, which controls the interior of the swamps. The southern end of the Neck is held by House Tully of the Riverlands through their redoubtable vassals, House Frey of the Twins. The Neck presents a formidable tactical obstacle to anyone planning to invade the North and was instrumental in holding off the invading Andals. However, it was not effective against airborne dragons, leading King Torrhen Stark, the last King in the North, to his decision to bow the knee to Aegon the Conqueror during the Targaryen Conquest.
In the books
In the Song of Ice and Fire novels the Neck was originally a much more pleasant location, but during the war between the Children of the Forest and the First Men it became a point of no retreat for the Children. When their formidable fortress of Moat Cailin looked like it might fall, the Children called upon the same sorcery they had used to shatter the Arm of Dorne centuries earlier to destroy the Neck and split Westeros in half. The sorcery was not effective and they merely flooded the Neck from coast to coast, creating bogs and swamps. However, the display was impressive and possibly killed many thousands of the First Men. It is possible that this display of power forced the First Men to make terms of peace with the Children, leading to the Pact and four thousand years of peace.
See also
Template:The North Template:The Riverlands