Elizabeth Andrews: extreme vocals on "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)
Cinzia Carraturo (Black Swan student special guest): lead and backing vocals live (2013)
Mattia Casabona: extreme vocals live (2020)
Eleonora Comba (Black Swan student special guest): lead and backing vocals live (2013)
Donald D. Docker: spoken voice on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012)
Douglas R. Docker: lead and backing vocals, spoken voice on all albums and live (2008 - now)
Göran Edman: lead and backing vocals, spoken voice on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012)
Kelly Massenz (Black Swan student special guest): lead and backing vocals live (2013)
Giacomo Mazzaro (Black Swan student special guest): backing vocals live (2013)
Tony Mills: lead and backing vocals on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012)
Serena Moine: lead and backing vocals on "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)
Lucia Partiti (Black Swan student special guest): backing vocals live (2013)
John Payne: lead and backing vocals on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012)
Anna Petracca: lead and backing vocals live (2013, 2019-2020), "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)
Valentina Procopio: lead and backing vocals on "The Heisenberg Diaries - Book A: Sounds of Future Past" (2016), live (2018), "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)
Andrea Rampa: spoken voice on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012), lead and backing vocals live (2013, 2019-2020)
Janthana Rodjakkhen: spoken voice on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012)
Elize Ryd: lead vocals on "The Heisenberg Diaries - Book A: Sounds of Future Past" (2016)
Jessica Scardina: lead and backing vocals live (2019-2020)
Amanda Somerville: lead and backing vocals on "The Mystic Technocracy - Season 1: The Age of Ignorance" (2012), "The Heisenberg Diaries - Book A: Sounds of Future Past" (2016) and "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)
Anneke Van Giersbergen: lead vocals on "The Mystic Technocracy - Season 2: The Age of Entropy" (2021)