The band returned to London, taking their first break from writing and touring since the recording of their debut album An Awesome Wave in 2011.

Drummer Thom Sonny Green released his solo debut album High Anxiety on 19 August 2016; keyboardist Gus Unger-Hamilton started Dandy Café, a pop-up restaurant in London Fields;[11] and lead vocalist Joe Newman took time off, during which he watched films.

[12] The band reconvened in a writing studio in North London to "take the bubble wrap off" their ideas for new material and eventually began recording for the album in August 2016.

[13] In November 2016, Alt-J traveled to Ely, a cathedral city in Cambridgeshire, to conduct field recordings of its organ and its boys' choir, which Unger-Hamilton was a member of during his youth.

[16] "In Cold Blood" features "interweaving vocals" underpinned by the "flashes" of a brass section and the "bleeping electronics" of a Casiotone purchased by the band for £1.05 on eBay.

Its lyrics are based on the plot of Richard Llewellyn's book How Green Was My Valley, about a small mining community in nineteenth-century Wales.

It is structured as a hymn and features the boys' choir and the organ of Ely Cathedral, which were added to give a "church feeling" to the song.

[31] On 29 March 2017, Alt-J premiered "In Cold Blood" on MistaJam's BBC Radio 1 show as his 'Hottest Record in the World'.

[38] Alt-J partnered with director Alex Takacs for a second time for the music video for "Deadcrush" and released it on 12 July 2017.

[39] On 19 May 2017, Alt-J performed "3WW" and "In Cold Blood" on the sixth episode of the fiftieth series of Later... with Jools Holland, accompanied by Ellie Rowsell on "3WW".

Released as a free digital download on Canvasback Music's website, the EP collects live recordings of "Deadcrush", "Hit Me Like That Snare", "Dissolve Me", "Every Other Freckle" and "Pleader" performed by the band during the North American leg of the Relaxer Tour.

[43] Artwork for Relaxer's cover and promotional material was taken from the 1998 PlayStation video game LSD: Dream Emulator by Japanese artist Osamu Sato.

[47] In The Daily Telegraph, chief critic Neil McCormick found it "deeply gorgeous and utterly baffling", labelling it "internet era pop" with its "barrier-free absorption of so many different musical styles a product of the computer recording technology it is created with.

"[1] Lisa Wright of DIY praised the album, stating Alt-J have "crafted possibly the most strangely original niche in modern music.

"[24] Kyle Mullin of Under the Radar praised the album's diversity: "This all-over-the-map approach makes Relaxer a bit dizzying and tough to digest at first, and yet you'll be immediately captivated and intrigued by its distinctive mix.

"[54] In a negative review, Jayson Greene of Pitchfork said, "The truth is that alt-J have never had an identity, really, apart from Newman's mangled lyrics and the fidgety, distracted arrangements of their songs.