Lynch
Here for the cookies and the tunes
I think it's 'hardcore' rap that is tainted with elements of horror, supernatural, etc themes ... but I could be wrong
Its not a term that gets thrown around a lot where I am. If it is as Phil said referencing rap music than obviously I am not going to have heard of that term because I don't tend to venture into realms of music I don't get into aside from some old school hip hop.
Insane Clown Posse would be probably the most known Horrorcore group. Here is a couple of more Horrorcore references, as requested by Miss Magic.
Boondox - Pray With Snakes
Esham - Dionaea Muscipula
I do like the band name Boondox......was kinda expecting a southern influence on that one, but nodda