The Vaughns were the family who possessed Hergest Court for most of its history and "Black Vaughan" is supposed to be the ghost of Sir Thomas, who was killed in 1469 (his effigy is at nearby Kington church). Another candidate is a different Sir Thomas, beheaded in 1483 as a traitor to Richard III.
Either way, Black Vaughn is a malevolent entity who is said to appear in various forms - including that of a bull. He roamed the nearby countryside, terrifying women by leaping onto their coaches as they rode home. He is said to have left physical impressions in the ground after his visitation in the form of bare patches in the grass.
Similarly, a black dog is said to accompany him or roam the grounds by itself - supposedly the spirit of Sir Thomas' favourite hound. In fact, Arthur Conan Doyle himself stayed at the hall as a guest of a later (friendlier!) generation of the Vaughans and it is commonly believed that the story of the ghostly dog served as the inspiration for the Hound of the Baskervilles.
According to legend, Black Vaughan was actually laid to rest by a team of 12 people carrying crucifixes and - oddly - a newborn baby. Reading passages from the bible and praying fervently, they are said to have 'laid' his spirit to rest.
Despite this "exorcism", belief that Black Vaughan still haunts the hall remains alive to this day.