Founded in the 8th century, the Abbazia di San Vincenzo al Volturno is famous for its cycle of 9th-century frescoes by Epifanio (824–842) in the crypt. The original monastery was sacked and burned by the Saracens in 881 and subsequently rebuilt on the opposite side of the river; the crypt was miraculously spared. Surrounding it, the evocative archaeological area is also worth visiting.
The crypt is only open for short visits at certain times of day (see the website).
The abbey was rebuilt in the 10th century and was one of the foremost monastic and cultural centres in Europe for a time. It's still home to a small community of Benedictine nuns. Badly bombed in World War II, most of the current structure was rebuilt in the 1960s. The complex and ruins are a 30km drive northwest of Isernia, near Castel San Vincenzo.