The following creators are not necessarily mainstream in their music or their art. But all are insanely prolific (sometimes literally) and driven by darkness.
All of them push their creative abilities to the extreme to exorcise their own personal demons, be they mental illness, emotional abuse, or a fear of the unknown. All have carved out a niche in the avant-garde or outsider arts, often combining music, prose and art as an all-inclusive creative statement.
While their efforts might not receive the attentions of, say, Lars Ulrich's lavish fine-art habit, their output deserves no less respect and attention.