Mariusz Duda (born 25 September 1975) is a Polish musician and composer best known as the bassist and vocalist in the band Riverside. He is also a former member of Xanadu and did guest vocals for Indukti's album S.U.S.A.R. He released his debut solo album under the moniker Lunatic Soul on the Kscope label in October 2008. Mariusz released his second album under the name of Lunatic Soul in October 2010. Both albums are known for the absence of an electric guitar.