Shokushuu no Miko ~ Kinjirareta Shitei Shokushu Koukan

Takeru was cleaning the shrine as he did each morning when he stumbled and crashed into a tree. The impact revealed an old vase-like encasement... a mysterious vase with an ancient seal that was now broken! A multi-limbed horror of Nyarlathotep sprawled out into freedom, insane with rage, not caring who had sealed it away... it seized Takeru and his buxom step sister, shrine maiden Sakuya...!

[Edited from DLSite English]