Complex sentence: Vocalist/bassist Helmut Köllen (performing on the band's most successful albums, "Illusions" and "Spartacus") left the band after "Spartacus" to pursue a solo career.
Split into simpler sentences: Vocalist/bassist Helmut Köllen (performing on the band's most successful albums, "Illusions" and "Spartacus"). Köllen left the band after "Spartacus" to pursue a solo career.