I Found the Elixir of Immortality

By chance, I rented a weird two-bedroom apartment. In this two-bedroom apartment, an unexplainable mysterious event happened. In order to find the answer, I conducted a series of explorations. Finally, I discovered that there really is an immortal person in this world, and this immortal person is the landlord's grandmother. In order to solve the mystery of her immortality, next... This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.