A semi-Gothic manga created by Kaori Yuki and published in Hana magazine by Yume Comics.
Angel Sanctuary begins by telling the story of Alexiel, the organic angel. Alexiel is in love with God, but God is in love with her twin brother Rociel, or Roziel, the inorganic angel. After God's disapperance from heaven Alexiel invades Atzuiluth, or heaven with a group of demons.
Alexiel seals Rociel in Earth, but in turn Alexiel is sealed into a crystal, her "true" body is always to be sealed away. Alexiel's soul is always afterwards reborn as a human, and to be punished for her rebellion to God, her incarnation always ends with a tragic and painful death.
In the present Alexiel is reborn as the troubled boy, Muduo Setsuna. Blessed and cursed with strange powers Setsuna tried to lead a normal life, depsite the fact he's in love with his younger sister Sara.
Katan, one of Roziel's followers creates a video game called Angel Sanctuary, which sucks the energy out of the players and passes it to Roziel, slowly awakening him from Earth.
After Roziel awakens all hell breaks lose for Setsuna, he discovers his orgins, and the series continues from there.