The opening track of the album "Kveikur" seems to me like metal rock compared with the average volume and tempo of most Sigur Rós songs. I like very much its inner tension, something between math-rock and a horror film soundtrack. And it also has an ejoyable sung line... well, not exactly what I happen to sing in my bathroom, but surely well written and well sung. It's a highly creative piece of music, including electronic and acoustic sounds so perfectly mixed that you couldn't tell a violin from a synth.
A suggestive frame from the official video of "Brennsteinn".
And the dark, aggressive mood of "Brennisteinn" strikes me, as I find here both sides of the band: the ethereal, spacey one and the upsetting, experimental one. More than this: I could listen to this song one hundred times and still each time I'd find something new, a hidden sound, an underlying line, a secret passage. Last but not least, I think Andrew Huang's official video perfectly captures the atmosphere of the track. Pure post-prog pleasure for your eyes and ears.