Kasvot Växt are about as obscure as a band can get. The Scandinavian prog rockers’ legendary lone album í rokk was released in the early ’80s “on a label so small it was essentially a private press,” according to an Allmusic review. The band’s four members met while participating in a shadowy scientific research project, and quickly disbanded after í rokk’s release. As fate would have it, their label soon went under as well, and in twist that could have been designed to make collectors salivate, most copies of the record were apparently lost in a warehouse fire.

Even when they were still around, they seemed willful about making themselves inaccessible, singing in multiple different languages (each member hailed from a different Northern European nation) and including no information about who played what in the liner notes. Our best information about what í rokk even sounds like comes from a 13-year-old blog post on the website of beloved noncommercial radio station WFMU, which describes a copy that turned up at the station one day long ago, became a favorite there, and disappeared after a “certain now ex-DJ” fell so hard for the album he decided to steal it for himself. There were homemade synths, chanting, inscrutable Norwegian lyrics about “the cubes.” The station’s meticulously archived playlists show that several DJs managed to play í rokk on the air before it disappeared.

That’s what Phish want you to think, anyway.

At a show in Las Vegas last night, the Vermont jam institution announced it would be continuing its on-again-off-again tradition of covering a classic album in its entirety for Halloween. This year it would be Kasvot Växt’s í rokk, a record that would be understandably unfamiliar to almost everyone in attendance. That’s because it isn’t a real record, or even a real band. All of the biographical information I’ve included above really did come from the sources I cited, but they were clearly all in on the joke.

WFMU’s breathless blog post about Kasvot Växt, though it is backdated to 2005, actually appeared on the site much more recently, possibly within the last 24 hours, according to an Archive.org search. The Allmusic review is likewise brand new. And so is a supposed interview from 2006 with band member Georg Guomundrson, who has long since moved on from his rock’n’roll days, on the music site Perfect Sound Forever. (“I am not certain I have a copy. If I do, it’s with my parents. I have no need to listen to that.”) Several WFMU playlists were doctored after the fact: if you go back and listen to the shows in question, there wasn’t any actual Kasvot Växt played.

What we have here is an extremely well-executed gag from Phish, with the help of several co-conspirators. Seeding stories with Perfect Sound Forever and WFMU—exactly the sort of places that would be genuinely interested in this kind of rock esoterica if it were real—was particularly inspired. According to a review of last night’s show from the website JamBase, what the band actually played was “an ambitious suite of original Phish music played in the style of an ’80s prog-rock band yet still had elements of the Vermont-birthed quartet’s distinctive sound.” Maybe the process of coming up with a mythology for a fake band, and then writing music as that band, was a way to break out of their music-making routine. Or maybe they just thought it would be fun.

If you want to know what Kasvot Växt sounded like, you can surely find some bootlegs from the Phish show floating around somewhere. Or you can just listen to one of the real old Scandinavian psychedelic bands their convoluted backstory seems to be winking at. May I suggest Träd, Gräs & Stenar?