Eien no Aselia -The Spirit of Eternity Sword- (永遠のアセリア-The Spirit of Eternity Sword-) is a Japanese visual novel developed by CyberFront, Nippon Ichi & Xuse. While the anime adaptation aired from June 24, 2005 to March 24, 2006 by Studio Matrix and Japan Home Video, a fan translation was created by Dakkodango Translations and also released by JAST Densestu.
Yuuto has been taking care of his step-sister Kaori since the passing of both of their parents. Despite their hard life, they remain happy together - at least until the day they are transported to a parallel world that they will later come to call Phantasmagoria...
In this world where he can't even understand a single spoken word, his sister is taken prisoner and her captors, calling him an 'Etranger', force him to fight wielding a sword that tries to control his mind instead...
Although he first accepts this fate only to protect Kaori, with passing time Yuuto starts to care for the fate of the 'spirits', the girls that fight alongside him, and other inhabitants of this world as well...