"Brennisteinn" (Icelandic for "Brimstone", pronounced /ˈprɛnːɪˌsteitn/) is a song written and recorded by Icelandic post-rock band Sigur Rós for their seventh studio album Kveikur. It appears as the opening track on the album. "Brennisteinn" was released as the lead single from the album on March 25, 2013, after the album's official unveiling three days earlier.

"Brennisteinn" was released to overwhelmingly positive reception. Ian Cohen of Pitchfork compared the track to the sounds of the band's second and third studio albums Ágætis Byrjun and ( ), stating that "Though their earliest work constitutes some of the most beautiful and ethereal music of the past two decades, Ágætis Byrjun and ( ), are heavy records with doom-friendly song lengths and tempos. "Brennisteinn", then, is a big payoff in that regard." He further calls the track a "heaven/hell juxtaposition" that "feels like the most logical step imaginable for Sigur Rós in light of last year's Valtari, a Calgon bath of a record that suggested that the band had run out of ideas." Grayson Hale of Sputnikmusic stated "As just a small taste of what's to come, "Brennisteinn" certainly achieves its goal of whetting the appetite." While Hale also makes a comparison to 2002's ( ), he writes "Although no two albums of theirs sound exactly alike, this is the first time we are really hearing something of a reinvention from the group."

Jake Jenkins of the Sanctuary Review noted of the band's approach of a more "aggressive" sound, but in allusion to this he wrote that "aggressive is an understatement." He describes the track as "a menacing, lurching beast of a song that will rattle you to your core." He criticizes Jonsi's vocal performance by stating that "Jonsi's vocals typically stay in his lower register but they still sit high above the rest of the music throughout the track, though their atmospheric texture lends itself perfectly to the song, and when he does get into that upper register they don't sound out of place in the least bit." However, he goes on to give the overall song a positive review by writing "Over the course of the 7 minutes, Sigur Rós prove that they have not only stayed true to their word of exploring a more aggressive sound but proved that they could pull it off in ways that exceed all expectations. Considering they were recently reduced to a trio, the fact that they sound this huge is impressive. If the rest of Kveikur is along the same lines as "Brennisteinn", we may be getting the best Sigur Rós album since ( ).

...
Wikipedia