Walls of Night
The Walls of Night were the extraordinary walls that surrounded Arda in ancient times, beyond Ekkaia, the Encircling Sea. In the west and east of the World, Ekkaia was wide, and the Walls were a great distance from land. In the north and south, however, the Encircling Sea was much narrower. This was how Melkor returned into the World during the time of the Lamps of the Valar, coming secretly over the Walls of Night into the north of Arda, and building there his fortress of Utumno beyond the knowledge of the Valar.