Complex sentence: He is the drummer of South Korean rock band CN Blue, which debuted in January 2010 in South Korea before that they debuted as indie band in August 2009 in Japan then made major debut in October 2011 in Japan.
Split into simpler sentences: He is the drummer of South Korean rock band CN Blue, which debuted in January 2010 in South Korea. Before that, they debuted as indie band in 2009 in Japan then made major debut in October 2011 in Japan.