Andalos is a region on the continent of Essos, located on the coast between the Free Cities of Pentos and Braavos, west of the Rhoyne River. It is the ancestral home of the Andals, an ancient people who, according to tradition, invaded Westeros six thousand years ago and are the ancestors of most of the people living in the southern half of the continent.
In the books
In the Song of Ice and Fire novels, Andalos was once a powerful kingdom extending across much of north-western Essos west of the Rhoyne River, including the Velvet Hills and the Flatlands. It was a contemporary of the early Rhoynar kingdom, and according to Rhoynish histories the Rhoynar taught the Andals how to forge metal (disputing Andal histories, which insist they were directly taught by the Smith). The area was originally home to a race of hairy men who were related to the men of Ibben. The Andals displaced them through warfare. The Faith of the Seven was born in Andalos when the Father gave Hugor of the Hill a crown made of seven stars, whilst the Maid gave Hugor his wife, who bore him forty-four sons.
Some time later, the Andals abandoned the region to invade Westeros across the Narrow Sea. They overran and conquered most of the continent, leaving Andalos deserted, and various other migrations of different peoples moved into the region over the course of the subsequent six thousand years. The Valyrians established the city of Pentos there millennia later, in the south of the region. The northern coast became part of the Braavosian coastlands.
The term "Hills of Andalos" has gained some traction in fan circles, as a term originating from George R.R. Martin reading a chapter from A Dance with Dragons some years ago. However, in the published novels the term "Velvet Hills" was used instead.