The Proud Fallen Angel is the first episode of the Seven Mortal Sins anime.
Freshly cast out of Heaven, the former angel Lucifer makes a brief stop in a high school church on Earth, where she encounters a girl named Maria Totsuka before continuing her fall to Hell. Once there, she is met and fawned over by Leviathan, an aspiring minor Demon Lord, and challenged to combat by Satan and Belial, two Demon Lords of the Seven Mortal Sins and members of the ruling council of Hell who accuse her of intending an invasion of their territory.
Despite gaining the upper hand initially, Lucifer is eventually subdued by Belial and stripped of her angel powers and wings; but with this last link to Heaven gone, Lucifer transforms into a demon lord herself. Along with Leviathan, she returns to Earth and meets Maria again, whom she promptly stabs through the heart.
("Numbers indicate order of appearance")