Lilium was the opening song of Elfen Lied which is a Japanese manga series written and illustrated by Lynn Okamoto. It was originally serialized in Shueisha's Weekly Young Jump from June 2002 to August 2005, with the 107 chapters collected into twelve tankōbon volumes. Elfen Lied revolves around the interactions, views, emotions, and differences between human beings and the Diclonii, a mutant species similar to humans in build but distinguishable by two horns on their heads and "vectors", transparent telekinetically controlled arms that have the power to manipulate and cut objects within their reach. The series is centered on the teenage Diclonius girl "Lucy" who was rejected by human beings and subsequently wants revenge.
Also with the popularity of Elfen Lied, its theme song, Lilium, became more and more popular.
Lilium is actually famous as I should say, I liked it and it sounds better than the TV version. Hope you like it, too!