He spent a year composing the material and recruited various vocalists to tell his story, including Gamma Ray's Kai Hansen, Stratovarius' Timo Tolkki, and Within Temptation's Sharon Den Adel. The backing band featured guitarists Jens Ludwig (Edguy) and Henjo Richter (Gamma Ray), bassist Markus Grosskopf (Helloween), and drummer Alex Holzwarth (Rhapsody). The album, 2001's The Metal Opera, was filled with fantasy themes and anthemic, progressive pomp-metal. It was followed by 2002's The Metal Opera, Vol. 2, and Avantasia returned years later with the Scarecrow Saga, consisting of 2008's The Scarecrow and 2010's The Wicked Symphony and 2011's Angel of Babylon. Given the success of the latter two records' world tours--which were virtually completely sold out, Sammet got more ambitious, deciding to make Avantasia's even more epic in scale--no small feat consideirng what he'd already accomplished.

In 2013, Avantasia issued The Mystery of Time, a full scale conceptual collaboration with the Deutsches Filmorchester Babelsberg. The list of vocalists was impressive: Joe Lynn Turner, frequent collaborator and ex-Helloween frontman Michael Kiske, Saxon's Biff Byford, and Magnum's Bob Catley among them (sometimes two and three at a time). The album charted number two in Germany, and in the top ten across Europe. It also entered the US charts at number nine--the first of the band's recordings to do so. The outfit undertook another five month tour, playing three hour shows and in sold out venues as well and at festivals in Europe, South America, Japan, Russia and Canada, culminating in a headline postion at the Wacken Open Air Festival, a performance Sammet claimed was Avantasia's last.

The claim was only in reference to live performances as The Mystery of Time left open-ended question in its narrative.

In January 2016, Avantasia released the sequel, Ghostlights. Many of the same singers returned and Sammet added to their numbers with appearances by Geoff Tate, Dee Snider, den Adel, and Ronnie Atkins, to name a few. ~ Steve Huey