Complex sentence: Viking did not have an official drummer when they began work on "No Child Left Behind" in 2011; the drums tracks on the album were played by Gene Hoglan (also a member of Dark Angel).
Split into simpler sentences: Viking did not have an official drummer when they began work on "No Child Left Behind" in 2011. But the drum studio tracks on this album were played by Gene Hoglan (also a member of Dark Angel).