Percussionists in LED-rimmed top hats bang on drums. Inflatable, insectlike creatures gyrate in the crowd. Multicolored lasers pierce a layer of fog. Every performer wears a mask. In short: If Mad Max merged with The Phantom of the Opera, the result would be an Itchy-O concert.

An underground music favorite, Denver’s Itchy-O is well-known for its blatantly bizarre live performances. Other details about the ensemble are difficult to come by, though. Born in 2009, the group has an unidentified founder and sixtysomething anonymous members who play percussion, guitar, and cymbals—and shroud themselves in elaborate, often creepy costumes. Itchy-O’s founder-who-cannot-be-named says the group creates otherworldly environments to get audiences out of their heads. “There’s a paradox where kind of terrifying stuff is happening, but then people are like, ‘Why am I dancing?’  ” he says. “They get caught up in the rhythm and joy of it all.”

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But the music and dance collective’s most distinctive feature might be its devotion to immersive art. Well before Meow Wolf popularized the trend in Denver, Itchy-O began exploring how audience members could add a sense of co-creation and unpredictability to its productions. At one show, for example, Itchy-O transported hundreds of fans who’d bought “mystery tickets” from one venue to another and then put them in costumes before feeding them through a 40-foot tube into a larger, unsuspecting crowd. And while all Itchy-O gigs break down the barrier between spectators and performers, Hallowmass—which celebrates its 10th anniversary this month—is the most interactive of all.

The annual, Halloween-adjacent event traces its origins to a 2014 trip Itchy-O made to Tucson, Arizona, to perform during the All Souls Procession—a Día de los Muertos–inspired parade that honors the dead. “We asked ourselves: What does Denver have that is comparable?” Itchy-O’s founder says. The group came up empty-handed, so in 2015 Itchy-O put on its first Hallowmass, which combined the collective’s penchant for partying with a more solemn ritual: Showgoers were asked to place items that represented people or things they wanted to honor or release from their lives onto an altar. “There were some pretty heavy things, like dog tags, baby shoes, and divorce papers,” the founder says.

Over the past decade, the typically sold-out Hallowmass performances have grown more ostentatious, with an entire folklore spinning up around the event—including an origin story for those insectlike creatures and a dedicated subsect of fans, dubbed the Āoth, who nerd out on the mythology and learn cryptic hand movements that coordinate with Itchy-O songs.

But for anyone getting cult vibes, Itchy-O’s founder maintains that the event is all in absurdist fun. Though newcomers should arrive with an open mind (and possibly something to leave at the altar), they aren’t pressured to interact with performers. “We pride ourselves on meeting people where they’re at,” he says. The enigmatic founder wouldn’t say what new spectacles the band will unveil at this year’s Hallowmass shows (October 31, November 1, and November 2 at Truss House in RiNo; tickets start at $40), but he promised this: Like all Itchy-O performances, “it’ll first and foremost be a big ass-shaking event.”