The image of the lovely Lady Sybil, tragic youngest daughter of the Crawley family on Downton Abbey is likely to go a long way towards reviving this almost forgotten name, off the list since 1966 and most popular in the 1920's and thirties. Sybil is currently Number 245 on Nameberry;
Sybil is the most common spelling of a name which the ancient Greeks used as the generic word for a prophetess--a woman who claimed to be able to interpret the wishes of the gods through their oracles.
Benjamin Disraeli wrote an influential political novel titled Sybil, which increased the popularity of the name in Victorian times. Later, the name was severely undermined by the eponymous book and 1976 movie based on the character of Sybil-- played by Sally Field-- whose multiple personalities have now been confessed as fake. This is not to mention the harridan wife on the classic British sitcom Fawlty Towers. There is also a Sybil in Oscar Wilde's The Picture of Dorian Gray and Sybill Trelawney is a witch and (fittingly) seer in the Harry Potter books, a Professor of Divination played on screen by Emma Thompson.
Cybill Shepherd, on the other hand, bears an invented name combining those of her Uncle Cy and Dad Bill.