Complex sentence: He was sent to Theresienstadt at the age of 15, where he was the violinist chosen to perform in Viktor Ullmann's opera "The Emperor of Atlantis or The Disobedience of Death", although the opera was rehearsed in 1944 at Theresienstadt, but never performed.
Split into simpler sentences: He was sent to Theresienstadt at the age of 15, where he was chosen to perform in Viktor Ullmann's opera "The Emperor of Atlantis or The Disobedience of Death". The opera was rehearsed in 1944 at Theresienstadt, but never performed.