The cover image is a detail from the right (Hell) panel of The Garden of Earthly Delights, a triptych painted in 1504 by Hieronymus Bosch, part of Madrid's Prado permanent collection. The original idea of using this painting for Into the Pandemonium's cover came through Martin Ain. The LP's inner sleeve was the Les Edwards Tombworld (1980) painting. As to how the band's music can be linked their album art, Tom Warrior explained:
In Warrior's recollection, three quarters into production Celtic Forst were forced, by Noise, to do some dates with Anthrax. If they didn't, the label would cut short the funding for Pandemonium. In his defense, label boss Karl-Ulrich Walterbach stated that he simply offered the tour and never pushed it. He knew the timing wasn't ideal for the band. One of these dates was the 1987 edition of the Aardschokdag, a Dutch annual heavy metal festival. Frost played alongside Anthrax, Metal Church, Crimson Glory, Lȧȧz Rockit and headliners Metallica on February 8.
After the album's release, American guitarist Ron Marks was invited to join the band, in part to bolster its live sound.
Celtic Frost did a brief English tour on late October, with Kreator and Virus. They then flew to the United States, to be the opening act to the December leg of Anthrax's Among the Living tour. Frost caught the thrash explosion in the US. In a few months, Anthrax went from playing to 500 people per show to 7,000 or 8,000 every night, without radio play or MTV exposure.
After Into the Pandemonium, Celtic Frost became one of Noise's bestsellers. By the end of the year, the band's third album had sold 100,000 records worldwide. Alongside the 250,000 sold by Running Wild's Under Jolly Roger and the 500,000 sold of Helloween's Keeper of the Seven Keys: Part I, 1987 helped usher a new era for the German label. There were a number of factors that aided this turn of events: SPV's European distribution network, RCA's promotional push behind Helloween and an enthusiastic metal press.
Thomas Gabriel Fischer finally performed Celtic Frost's requiem at Roadburn 2019 with Triptykon, along with the Metropole Orkest. Into the Pandemonium's "Rex Irae" is the opening part; the third, concluding part—"Winter (Requiem, Chapter Three: Finale)"—can be heard on 2006's Monotheist. The second, long-missing second part ("Grave Eternal") was never officially released until these performances. A full, live rendition of the entire piece has been released.
Malcolm Dome called Into the Pandemonium both "metal's most visionary album" and an "avant-garde metal masterpiece". Tom Warrior himself said that it is "the band's most important release". AllMusic reviewer Eduardo Rivadavia considered Into the Pandemonium "one of the classic extreme metal albums of all time". In 1991, Eddy ranked Into the Pandemonium at number 108 in his list of the 500 best heavy metal albums ever.
Into the Pandemonium had a decisive impact on the emerging gothic metal scene of the 1990s. Paradise Lost collectively held the album in high esteem. "It made [us] what we are", said Mackintosh, "and so many other bands [...] too". Rhythm guitarist Aaron Aedy pointed out that Pandemonium's use of orchestration inspired their sophomore album, Gothic. My Dying Bride's Andrew Craighan revealed that adventurous use of violins on Into the Pandemonium encouraged MDB to do the same. Craighan felt something like "we can do that if they're doing it". Moonspell's Ribeiro found Pandemonium "groundbreaking and inspiring". He preferred Celtic Frost's unorthodox approach to heavy metal "than to be bound to an unwritten book of Underground laws to please others instead of our artistic hunger". This particular record made them deepen the connection with their Middle Eastern and African musical heritage, a big part of the folk music from their native Portugal.
Warrior's "goth" crooning on Pandemonium also prove influential to gothic metal pioneers. Paradise Lost singer Nick Holmes said "Mesmerized" was his second favorite Celtic Frost song, partly because of how Warrior sang on it. Anathema's Vincent Cavanagh, later on, would borrow Warrior's moaned-style singing on "Mesmerized" for The Silent Enigma's title track.
Celtic Frost's third album also had a lasting influence on symphonic metal. Therion mainman Christofer Johnsson, in particular, frequently acknowledges its importance. In 2021, it was elected by Metal Hammer as the second best symphonic metal album of all time.
