Heavy metal music developed in the late 1960's with groups such as Black Sabbath and Led Zeppelin. From the origins of this new style of music emerged a huge sub-genre of music. The lyrics tend to focus on dark or depressing subjects, including death, war, paranoia and personal trauma. Some extreme forms often focus on aggressive or gory lyrics, while others focus on more philosophical themes. A common theme in most metal is the idea of doing what you want, despite what the world says.