Dansho no Grimm IV Nigyouhime (Part 2)

is the fourth volume of the Dansho no Grimm light novel series, written by Gakuto Coda and Illustrated by Mikazuki Kakeru. It was released under the Dengeki Bunko Imprint in April 2007.

Synopsis
On the eve of the fiancée's fiancée's seventh anniversary, the tragedy that traces the story of the mermaid princess spreads throughout the seaside town visited by Aoi and his friends. And when the truth of the mermaid princess who invites the chain of death is revealed. Nightmare fantasy new fantasy presented by a genius, Act 4!

(source: Bookwalker JP - Translated)