dunno how much you consider Traumatic Voyage as "outsider metal" but they were pretty ahead of their time with their weirdo psychotic prog death-cum-black metal. Its very technically proficient, albeit scrambled in approach and all over the place sound wise. I remember reading Astorian having mental illness (I think schizophrenia??) and that attributed a lot to the lyrical and sound theme.
Regardless, really fucking crazy stuff.