A biblical demon. He was mentioned by Rev. Hoadley in his sermon against the close presence of Satan in Dunwich shortly before he disappeared.
In Hebrew mythology, a satanic personification of wickedness and ungodliness alluded to in the New Testament (II Cornithians 6:15). In Milton's Paradise Lost, one of the fallen angels who rebelled against God. [Hebrew beliyya'al "unselessness" : beliy, without + ya'al, use.]
("The Dunwich Horror," "The Hound," "The Shadow Over Innsmouth")