A storyline like this seems like an excuse to make a horror manga. While this sounds like it’s actually good, it’s not. The mangaka seems to throw the plot into different directions.
The characters? Not even. Of course the female character is a damsel-in-distress (the only thing I remember about her is that she screamed almost every panel she was in, thankfully I liked what the mangaka did to her eventually). Of course the main character (male, even) has to save her often. The character development is very minimal. All I remember that the main character seemed to be slightly scarred by the incident.
The art was nothing really impressive, though it was very fitting for a horror manga. I remember there were so many details on the strange monsters, though there was nothing memorable about the details on the characters and on the backgrounds.
I came for the bonus story though, the Enigma of Amigara Fault. Now that is more considered to be a horror manga despite being a little cliched. 4/10 overall on Gyo though.
