Vergil is an alternative spelling of Virgil, the name of the Roman poet who wrote the Aeneid, Rome's national epic, around 2,000 years ago. He is also the guide who leads Dante through Hell in the Divine Comedy, which is why the name turns up so often in later literature. The two spellings point to exactly the same man. Virgil is far more common in ordinary English writing; Vergil is preferred by many classical scholars because it is closer to the Latin form of his name, Vergilius. So the spelling you choose quietly signals something: Vergil looks academic, Virgil looks normal. Neither is wrong. As a proper noun it takes a capital letter and no article — you say "Vergil wrote," never "the Vergil." The adjective from it is Virgilian (or Vergilian), meaning in his style — grand, ordered, full of fate and duty. The name is also used today as an ordinary given name for men, almost always spelled Virgil.