A faerie radhe is a progress, when the sidhe rode across the land. Such a progress would call other sidhe who would join with their horses and hounds, and lesser fey would be drawn to it to march together. Even humans could be drawn into it sometimes.
It was pointed out that there had never been a faerie radhe on American soil, that they lost their horses and the ability to call the folk to them. In Swallowing Darkness, however, according to Rhys, a radhe occurred when Hettie's body was transported home. "Every brownie on American soil came out to receive Hettie. They took her from us, and keened for her."