What makes Hikari no Senritsu by Kalafina notable?

'Hikari no Senritsu' played a vital role in popularizing Kalafina's unique sound, characterized by its blend of classical and pop influences. It was used as the opening theme for the anime 'Kara no Kyōkai', which contributed to both the single's commercial success and the rising popularity of anime music during this era. Released on January 20, 2010, 'Hikari no Senritsu' is Kalafina's debut single following their formation in 2007. At this point, they were establishing their identity within the J-pop scene, building on their previous work with composer Yuki Kajiura. The single marked a significant step in their growth as artists, transitioning from background vocalists to leading performers.