Seijaku wa Yami no Shirabe

Few years after the World War II, the protagonist receives a letter naming him as heir to the inheritance by his sick biological father - the fact that he has no memory of, yet which his current parents reluctantly admits as true. He returns to his hometown with intention of rejecting the offer, and meets a relative who still remembers him from her childhood. As the stay unintentionally grows longer, sequence of strange events begins to take place against him...