AllMusic wrote of the album's style: "Satyricon still get a lot of flack every time a new album is released that doesn't conform to the outdated, rustic-necro-kvlt brand of Norwegian black metal they played in the early '90s (alongside Darkthrone, Ulver, Enslaved, etc.) Their sixth full album, Now, Diabolical is no exception, and it hardly out-shocks its similarly unconventional predecessor, 2002's Volcano; [it] just brings the band's present mentality of addition by subtraction to its next logical step", describing, "[...] cuts like 'A New Enemy', 'That Darkness Shall Be Eternal' and the title track find the core duo of Satyr and Frost seeking simplicity above all else; consistently shunning complicated arrangements and overbearing displays of musicianship to exercise an almost industrial sense of discipline whilst executing their hypnotic riffs, sinister melodies and static tempos."
Blabbermouth.net opined, "Songs like the title track and "K.I.N.G." are all about the aforementioned groove. The driving rhythms and Satyr's vocal patterns are downright catchy. This is not about virtuosity or esoteric displays to prove an underground credibility. It is about hard driving tempos and head banging with a snarl on your face."
On the vinyl version, as well as on the Scarecrow Records limited edition CD of the album there is a bonus track "Storm (Of the Destroyer)". Two singles were released from the album: "K.I.N.G." and "The Pentagram Burns". Both with videoclips.