While still in Blind Guardian, he formed Savage Circus with friend Piet Sielck. Piet introduced him to Persuader vocalist Jens Carlsson and guitarist Emil Norberg, who completed the band's lineup. They released their debut album Dreamland Manor in August 2005. Due to health problems causing him to miss too many gigs, Thomen left the band on 17 August 2007 but came back to the line-up in 2012.
Thomen is known for his fast and technical drumming skill; especially on the Blind Guardian album A Night at the Opera where he mixes the traditional intensity and speed of the band's older style, with new, more technical and progressive elements. He is a user of Premier Drums, Paistecymbals and Vic Firthdrumsticks.