Complex sentence: The Longships, a group of rocky islets are just over a mile offshore of Land's End, and the Isles of Scilly lie approximately southwest--the mythical lost land of Lyonesse (referred to in Arthurian literature) was supposedly located between Scilly and the mainland.
Split into simpler sentences: The Longships, a group of rocky islets are just over a mile offshore. The Isles of Scilly lie approximately southwest of Land's End--the mythical lost land of Lyonesse (referred to in Arthurian literature) was supposedly located between Scilly and the mainland.