Bies or bes ( ˈbʲes) was an evil spirit or demon

Demon

call - 1347 531 7769 for more infoIn Ancient Near Eastern religions as well as in the Abrahamic traditions, including ancient and medieval Christian demonology, a demon is considered an "unclean spirit" which may cause demonic possession, to be addressed with an act of exorcism...

 in Slavic mythology

Slavic mythology

Slavic mythology is the mythological aspect of the polytheistic religion that was practised by the Slavs before Christianisation.The religion possesses many common traits with other religions descended from the Proto-Indo-European religion....

. The word is synonymous with chort

Chort

In Slavic mythology , Chort is considered to be a demon of total evil, with horns, hoofs, skinny tail, and a pig-face. He is the son of the Slavic god Chernobog and the goddess Mara. In Ukraine, he is also known as haspyda, didko, irod, and kutsyi. In folklore, his physical appearance is same as...

.

After the acceptance of Christianity

Christianity

Christianity is a monotheistic religion based on the life and teachings of Jesus as presented in canonical gospels and other New Testament writings...

 the bies became identified with the devil

Devil

The Devil is believed in many religions and cultures to be a powerful, supernatural entity that is the personification of evil and the enemy of God and humankind. The nature of the role varies greatly...

, corresponding to the being referred to in Ancient Greek, as either daimon (δαίμων), daimónion or pneuma (πνεῦμα). For example, biesy (Russian plural of bies) is used in the standard Russian translation of Mark 5:12, where we have the devils entering into the swine in KJV.
Compare to the Ukrainian

Ukrainian language

Ukrainian is a language of the East Slavic subgroup of the Slavic languages. It is the official state language of Ukraine. Written Ukrainian uses a variant of the Cyrillic alphabet....

 bisy (used always in plural) or bisytysia (to go mad).

Examples in culture

• In Alexander Pushkin's The Tale of the Priest and of His Workman Balda
  The Tale of the Priest and of his Workman Balda
  The Tale of the Priest and of his Workman Balda is a fairy tale in verse by Alexander Pushkin. Pushkin wrote the tale on September 13, 1830 while staying at Boldino. It is based on a Russian folk tale which Pushkin collected in Mikhailovskoe early on...
  
  , there is a scene in which Balda has to force the "devils" (черти, Cherti) of the sea to pay an ancient rent, and interacts with an "Old Bies" who is their leader and his grandson.
• The original Russian title of Fyodor Dostoevsky
  Fyodor Dostoevsky
  Fyodor Mikhaylovich Dostoyevsky was a Russian writer of novels, short stories and essays. He is best known for his novels Crime and Punishment, The Idiot and The Brothers Karamazov....
  
  's novel known in its English translation as The Possessed is Besy (Бесы) (Russian plural of bes), i.e., more literally, The Evil Spirits.