Nicola Sapone, one of the leaders of the "Beasts of Satan" rock band, was sentenced to life behind bars for the 1998 murders of band singer Fabio Tollis and his girlfriend Chiara Marino.
The group had apparently believed that Ms Marino, who was stabbed to death under a full moon, was the personification of the Virgin Mary.
She was 19.
Mr Tollis, 16, took a fatal hammer blow to the head after trying to prevent her murder.
Sapone covered Mr Tollis's mouth with a chestnut husk to silence him and also slit his throat in what Italian media called "an orgy" of bloodletting.
The band members then pushed the dead bodies into a pit and urinated on them.
Four other followers of the band received sentences of between 24 and 26 years.
Their convictions followed the high-profile conviction of band leader Andrea Volpe, who received a lighter, 30-year sentence after leading authorities to the bodies and confessing.
“Beast of Satan” band members were also convicted for murdering Volpe's ex-girlfriend Mariangela Pezzotta in 2004.
She was shot in the mouth and her body mutilated.