Question: The album On the Turn was produced by GGGarth, who previously worked with what American heavy metal band from Los Angeles, California, formed in 1991?
Sentence (from "Omen (band)"): The band was formed in Los Angeles, California, in 1983, by lead guitarist Kenny Powell, previously with the band Savage Grace and signed on with Metal Blade in 1984, with whom they released their debut "Battle Cry" (which contained "The Axeman", which appeared on the soundtrack for the Heavy Metal themed 2009 video game "Brütal Legend").
Relevant evidence: No