Kiske's debut solo album Instant Clarity. Together with HammerFall he recorded a cover version of Helloween's "I Want Out". Hansen played the role of Regrin the dwarf in Avantasia's Metal Opera albums by Tobias Sammet. Later he was featured in the album The Scarecrow playing guitar in the song "Shelter from the Rain". He was also one of the guest musicians in Avantasia's 2010 world tour, which have put him to play live together with fellow ex-bandmate Michael Kiske, after more than twenty years since he left Helloween. After this tour it was announced, via Michael Kiske's official website, that Hansen would join his new band Unisonic, as they "instantly felt the magic of Helloween years again and had a lot of fun together on and off stage". In 2005, he toured with the young German power/Viking metal band Stormwarrior from his home town of Hamburg as lead singer, playing material from his classic Helloween debut album Walls of Jericho – the favorite album and greatest influence of Stormwarrior. "Stormwarrior featuring Kai Hansen" returned in 2007 to play at the Magic Circle Festival in Bad Arolsen and at the Wacken Open Air Festival. In 2008, Hansen played with Stormwarrior at Sweden Rock Festival 2008. In 2011, Hansen joined the hard-rock band Unisonic, as second guitarist and reunited with old friend and ex-Helloween vocalist Michael Kiske, who formed the band in 2009. The band released its debut album, Unisonic in 2012 and embarked on its first world tour. Hansen stated in an interview that despite being a member of Unisonic, he will not have less focus on Gamma Ray, and that the next Gamma Ray album should arrive in 2013. Unisonic's second album, Light Of Dawn, was released in 2014 and a European and Japanese tour followed. In 2015, following the Best of the Best Party Tour, Hansen hired Frank Beck as their second vocalist for the Gamma Ray. In 2016, Hansen released his first solo album titled XXX – Three Decades in Metal. The album was released via earMUSIC on September 16 of the same year. In 2017 Kai Hansen played minitour - four shows in Slovakia and Czech republic with the band Ravenclaw, with special set for Ravenclaw 15-Years Anniversary - Ravenclaw feat. Kai Hansen. On November 14, 2016, was announced the Pumpkins United World Tour - a concert tour of the German power metal band Helloween. Former members