Once Was Not is the first album to feature original vocalist Lord Worm since 1996's None So Vile, and would be his last with the band. It is also the first album not to feature original guitarist Jon Levasseur. Unlike previous albums, where Lord Worm's lyrical content consisted of gore, blasphemy, and sex, his lyrics on Once Was Not dealt with topics that affect the human race, such as war, famine, plague, and death.
A digipack edition was released, limited to 10,000 copies, which comes in an embossed digipack with an entirely gold-ink-printed booklet.
A video was filmed for the track "The Pestilence That Walketh in Darkness".