Produced by Ilya, Omer Fedi, Blake Slatkin, Jimmy Napes, and Cirkut and written by them alongside Smith and Petras, "Unholy" is a sexually charged electropop, dance-pop, and synth-pop song with choral and hyperpop influences.

The music video for "Unholy", directed by Italian-Canadian director Floria Sigismondi, depicts Smith and Petras performing at an erotic cabaret with burlesque dancers to a woman who follows her adulterous husband there.

[2][3][4] Petras agreed to work with Smith on "Unholy" after being sent a rough draft of the song and the two first met at Capitol Studios in Los Angeles a week later.

[2] "Unholy" was recorded in Jamaica and produced by Ilya, Omer Fedi, Blake Slatkin, Jimmy Napes, and Cirkut, all of whom co-wrote the song with Smith and Petras.

[1] It was first teased in mid-August through a clip on TikTok, which shows Smith and Petras dancing in a recording studio to a snippet of it that went viral and spurred a trend of sexually charged, thirst trap-style videos on the platform.

[11] A sexually charged[12] and campy[13] electropop,[14] synth-pop,[15] and dance-pop song,[16] "Unholy" marked a return to EDM and a shift from sentimental ballads to dance music for Smith.

[16] It features Gregorian choir-like vocals,[18][21] a "grinding" bassline,[22] a string section,[23] and metallic hyperpop-style synths and percussion popularized by Scottish musician Sophie and the record label PC Music.

[23] Chris Molanphy of Slate remarked upon the song's "sinister edge" and "high-camp energy" and compared it to "early-2010s peak-EDM era" pop music by artists such as Avicii and Kesha.

[24] Vulture's Jason P. Frank wrote that it was "a difficult song to get excited about" and "impressively un-tantalizing, the most basic kind of infraction", also asking, "Is this truly the best gender transgression we have to offer right now?

"[27] Slate's Chris Molanphy wrote that "Unholy" was "serviceable radio fodder" that was "almost conventional in its adherence to Top 40 trends" and that he "waver[ed] between delight and annoyance at [the song's] hook".

Petras's verse was praised by Dominguez, who wrote that she "plays her sugar baby role to perfection", though it was called "trite" by Mark Richardson of The Wall Street Journal and "unimpressive" by Riley Moquin of The Line of Best Fit.

[49] It also topped the charts in Singapore, New Zealand, Australia, Bulgaria, Canada, Greece, Ireland, Hungary, Lithuania, Malaysia, Netherlands, and Slovakia.

[10] Inspired by A Clockwork Orange and Bob Fosse, it was directed by Floria Sigismondi and choreographed by the French dance collective (La)Horde.

[60] In January 2023, Smith was the musical guest on a season 48 episode of Saturday Night Live hosted by actress Aubrey Plaza the week before the release of Gloria, where they performed "Unholy" in a top hat with devil horns and a large, pink, and ruffled dress designed by Tomo Koizumi that revealed Petras sitting inside of it; the performance also featured hooded dancers.

[61][62] Michael Cragg of The Face wrote that the performance "felt exciting" as "an emblem of Smith's journey from purveyor of middle England Marks & Spencers music ... to something, perhaps, more genuinely groundbreaking.

The Satan-themed performance showed Smith in red leather clothing and a top hat with horns, surrounded by pyrotechnics and backup dancers dressed similarly to Samara Morgan (Daveigh Chase) from the 2002 horror film The Ring, as well as Petras dancing in a cage.

[63] American conservative politicians and pundits, such as Ted Cruz, Marjorie Taylor Greene, and Matt Walsh, and online conspiracy theorists espoused the idea that the performance was a form of devil worship, with dozens of people filing complaints with the Federal Communications Commission (FCC) for that reason.

[67] A week later, they gave a performance of the song at the Brit Awards 2023 in an industrial set with black leather mechanic outfits, which was delayed due to technical issues and received 106 complaints to Ofcom.

[76] Lizzo's performance included a live band and a 30-second-long flute solo from her; Justin Curto of Vulture stated that her cover was "better" and "more soulful" than the original.