Wikipedia
Westernesse is a fictional kingdom in the Middle English romance of King Horn. It has been associated with various regions in the British Isles, among which are Isle of Man and the Wirral district in Cheshire.
Whether or not Westernesse should be identified with one or more real-world locations remains, however, a point of dispute, since the narrative contains little geographic detail, and only vaguely echoes historical events.
In King Horn, Westernesse can be reached by sea, and is ruled by King Almair. The throne eventually passes to Arnoldin, who is a loyal companion of the narrative's hero, Horn.
Many people today know 'Westernesse' as J. R. R. Tolkien's translation of Númenor, the name which he gave to one of the realms in his fictional world of Middle-earth. In one of his letters, Tolkien indicated that he had derived his translation from the name as it occurred in King Horn: