Complex sentence: Their first full length release was "Rituale Satanum" back in 2000, followed by "By the blessing of satan" in 2004, the latter is considered by many to be their opus magnum thus far.
Split into simpler sentences: Their first full-length release was "Rituale Satanum", released in 2000 and followed by "By the Blessing of Satan" in 2004. The latter is considered by many to be their opus magnum thus far.