An assistant to Johnathan's wife tells TMZ ... Johnathan passed Tuesday night while at home "cuddling" with his wife of 11 years, Anastasia Synn. We're told a hospice nurse had been staying with him for the last 3 weeks, and confirmed he had passed.

The former Las Vegas headliner, whose real name was Johnathan Szeles, had been diagnosed with congestive heart failure in 2007, and it's been a very tough road. The assistant says, "We have thought we lost him so many times that we never thought it would actually happen."

Johnathan was reportedly making up to $3 million per year at the peak of his Vegas fame when he had a residency at Bally's casino. His gonzo act featured some astounding, and downright shocking, moves ... including putting a spike through his tongue and sucking on his dangling eyeball.

The Detroit native gained a more mainstream audience after appearing on Criss Angel's reality show, "Mindfreak."

During a 2014 appearance, he announced he'd been told he only had one year to live ... but he was still around 5 years later when he launched a comeback tour.

Anastasia's assistant says they will have memorials for Johnathan in Vegas, Michigan and in L.A. they will hold the "Los Angeles Broken wand ceremony at the Magic Castle."

Johnathan was 63.