Aiyoku no Eustia is August's eighth game, preceded by Fortune Arterial and Yoake Mae yori Ruriiro na: Moonlight Cradle.

After the surface of the world succumbed to chaos centuries ago, the city of "Novus Aither" (ノーヴァス アイテル, Novasu Aiteru) is kept afloat in the sky solely by the continual prayers offered by the Saint.

[2] Years ago, when the Saint's prayer was interrupted, an incident known as the "Gran Forte" occurred, causing a portion of the land to fall and split the lower city into two, separated by a cliff.

Crime, famine, and disease run rampant, with "The Noncorroding Gold Chains", the ruling organization in the area specializing in the sex trade, keeping what little order that still exists.

In this "Prison" resides Caim Astraea, a freelance ex-assassin who does odd jobs for his friend, the head of "The Noncorroding Gold Chains", for money.

During one request, Caim finds Eustia, a winged girl who emits the pale purple light characteristic of the "Gran Forte" incident years ago.

A novel entitled Aiyoku no Eustia Auld Lang Syne, written by Okita Kazuhiko, was published by Paradigm on December 22, 2011.

On December 24, 2010, the first trial edition, Aiyoku no Eustia - Tech Gian Super Prelude, was made available on platform Windows, with an age rating of 15+.

The second one contained the ending theme "Shinainaru Sekai e" (親愛なる世界へ) by Ceui and the insert song "Close My Eyes" (クローズ・マイ・アイズ) by Ami Fujisaki.