Bilaris was an Age that served as a refuge from the fall of D'ni for a number of survivors, including Oma and Esel and their father.
The Age was found in a great house in the Jaren District during the First Restoration of D'ni. The clean state of the room where the Book was found gave Atrus and his expedition hope that they would find survivors there, but a subsequent exploration of the main island revealed no inhabitants. In fact, Oma and Esel's father moved everyone to the far side of one of the nearby islands where they could no be seen from the link-in point. He had feared that whatever destroyed D'ni would follow them to Bilaris, and forbade anyone from returning to the Cavern.