Elfen Lied has a rating of 18+ along with a thick, interesting plot.
To sum up, it’s total of 13 episodes consist of about 50% gore and nudity. However, the back story of the characters is told in a heart wrenching way. Some scenes are enough to make a grown person cry , Most of it also has to do with the strategic placement and perfect selection of soundtracks and music . The Opening theme “Lillium” is inspired by the biblical genre and is a personal favorite.
Elfin Lied is centered around a mysterious girl who narrowly escapes from a government laboratory and later gets shot in the head which causes a mental disability in her. As story develops, a couple of cousins find her wounded and unconscious on the beach and let her stay with them without knowing her real identity and thats when the real story begins!
The Anime and manga both have different endings so if you prefer to know the alternate ending, be sure to check out the other version.
Elfen Lied Library on Crunchyroll: http://www.crunchyroll.com/library/Elfen_Lied