Complex sentence: Jeff "Mantas" Dunn is one of the founding members of the thrash/speed metal band, Venom, with which he played as a guitarist from 1979 to 2004.
Split into simpler sentences: Jeff "Mantas" Dunn is one of the founding members of the thrash/speed metal band, Venom. He played with Venom as a guitarist from 1979 to 2004.