The Skystone is the first book in the Camulod Chronicles by Jack Whyte. The book tells the story of the men who become the grandfathers of Merlyn and King Arthur.
It is narrated by Publius Varros (grandfather to Arthur), who was a soldier in the Roman legion near the end of Rome’s presence in Britain. After suffering a grave injury in battle, he returns to his work forging weapons until Caius Britannicus, his former commander and future grandfather to Merlyn, comes to him and tells him the Romans are withdrawing and Britain will be defenseless. The men travel to Caius’ villa, where they unite all the nearby villas in a defensive system.
Whyte does a nice job of weaving in the magical and fantasy elements of Arthurian lore, and giving them a realistic context. Camulod (Camelot) is the fort Caius and Publius build at the top of the hill on Caius’ villa. Excalibur is a sword made from the stone of a meteor that landed in a lake, which explains the source of the legend that Excalibur came from the lake near Avalon. When Publius was testing the unusual stone, he molded it into the form of a woman which they dubbed the “lady of the lake”, explaining another legend.