The Isles of Antillia were located westwards across the great Western Ocean, far beyond the known realms of Hyboria. The Antillian islands consisted of seven large islands in the Western Ocean between Hyboria and a much larger continent still farther to the west called Mayapan. It took Conan the Cimmerian and his crew of adventurers greater than two months to navigate to the islands aboard the Red Lion when they sailed westwards from Tortage.

The capital of the Antillian kingdom was the city of Ptahuacan, where the dark wizard-priests of the Hierarch of the Sacred Mysteries of Xotli ruled over the people through fear, and conducted dark rituals of mass sacrifice their demon-god.

During the Hyborian Age, the Antillian islands were inhabited by a race of descendants from ancient Atlantis. The ancestors of the Antillians were the survivors of the Atlantean cult of Xotli, who had fled the destruction of Atlantis during the Great Cataclysm in a mighty fleet of flying ships. The original priests of Xotli ventured southwest from the doomed continent of Atlantis until they made landfall in the little known island chain that they named Antillia.

When the Atlanteans landed, they found a race of small brown, slant-eyed savages similar to the people of Mayapan. The Atlanteans easily conquered these natives and reduced them to the same state of slavery as the servants they had brought with them. During the millennia after the Cataclysm, the blood of the Atlanteans and of the aboriginal Antillians mixed until by the time Conan encountered them, the islands were inhabited by a single, mixed race.

Some mythical tales of Antillia were known in the kingdoms of Hyboria, but such was the belief that nothing existed across the Western Ocean that few ventured westwards and fewer, if any, ever returned. According to Sigurd Redbeard, the Seven Cities of the Antilles were thought to have walls made of golden bricks, and streets paved with silver, and temple pyramids of orichalcum, although this did not prove to be the case when the crew of the Red Lion travelled there.