Lilium is the opening theme song from the anime Elfen Lied. It was originally composed by Kayo Konishi and Yukio Kondou with lyrics based on Latin biblical texts.
I believe the original song was composed for either a piano quintet (2 violins, 1 viola, 1 cello, 1 piano) and vocals or a string chamber ensemble plus a piano and vocals. Thus, one of the challenges in reducing this score to for solo piano was to choose the which pitches to include in order to capture the fullness of strings, while at the same time keeping in mind the constraints of the pianist (e.g. maximum stretch, pedaling with minimal blurring). The melodic line often crosses under or is surrounded by harmonizing parts. One of the most difficult passages is at bar 9 (at 0:55 and at 2:23) where the middle triplet figures are shared by both hands.