This is may not be reliable but this is what WIKI says,

Horrorcore is a subgenre of hip hop music based on horror-themed lyrical content and imagery. Its origins derived from hardcore and gangsta rap artists such as the Geto Boys, which pushed the violent content of its lyrics further than most artists in the genre, as well as describing supernatural themes. The term horrorcore was popularized by openly horror-influenced hip hop groups such as Flatlinerz and Gravediggaz.
Contents

The stylistic origins of horrorcore can be traced to the Geto Boys' debut album, Making Trouble, which contained the dark and violent horror-influenced track "Assassins", which was cited by Joseph Bruce (Violent J of the horrorcore group Insane Clown Posse) in his book Behind The Paint, as the first horrorcore song. He said that the Geto Boys continued to pioneer the style with its second release, Grip It! On That Other Level, with songs such as "Mind of a Lunatic" and "Trigga-Happy Nigga."[1] Ganksta N-I-P's debut album, The South Park Psycho (1992), includes the song "Horror Movie Rap," which samples the soundtrack from the 1978 film Halloween.[2][3] Big L's debut single "Devil's Son" (1993) is considered horrorcore.[4] The group Insane Poetry, on its debut Grim Reality (1992)[5] and Esham, with Boomin' Words from Hell (1989), both incorporated horror imagery with its lyrics.[6] Kool Keith claims to have "invented horrorcore".[7] According to Icons of Hip Hop, "There is much debate over who coined the term horrorcore," but the word gained prominence in 1994 with the release of Flatlinerz' U.S.A. (Under Satan's Authority) and Gravediggaz' 6 Feet Deep (released overseas as Niggamortis).[2][8] [9][10]

The genre is not popular with mainstream audiences as a whole; however, performers such as Insane Clown Posse and Twiztid have sold well.[2] The genre has thrived in Internet culture and sustains an annual super show in Detroit called Wickedstock.[11] Every Halloween since 2003, Horrorcore artists worldwide get together online and release a free compilation titled Devilz Nite.[12][dead link] According to the January 2004 BBC documentary Underground USA, the subgenre "has a massive following across the US" and "is spreading to Europe".[11] Rolling Stone in 2007 referred to it as a short-lived trend that generated more shlock than shock.[13] New York Magazine put horrorcore in the spotlight by listing off the ten most horrifying horrorcore rappers.[14] Spin asked Violent J of Insane Clown Posse to list off his favorite horrorcore songs. Songs included, The Dayton Family's "What's On My Mind", Bone Thugs-N-Harmony's "Mr. Ouija", Necro's "Billie Jean 2005", and Michael Jackson's "Thriller".[15]
Characteristics

Horrorcore defines a style of hip hop music that focuses primarily around horror-influenced topics that can include Hallucination, satanism, self-harm, cannibalism, suicide and murder. The lyrics are often inspired by horror movies over moody, hardcore beats.[16] According to rapper Mars, "If you take Stephen King or Wes Craven and you throw them on a rap beat, that's who I am."[17] Horrorcore was described by Entertainment Weekly in 1995 as a "blend of hardcore rap and bloodthirsty metal."[18] The lyrical content of horrorcore is sometimes described as being similar to that of death metal, and some have referred to the genre as death rap.[19] Horrorcore artists often feature dark imagery in their music videos and base musical elements of songs upon horror film scores.[19]