Zoufu no Hanataba

The story begins when middle schooler Ryuunosuke visits his grandmother's house on an island. As Ryuunosuke walks past the stairs leading to the second floor, a strange, plant-like thing extends down from above. Intrigued, Ryuunosuke tries to climb the stairs, but his uncle stops him with an uncharacteristically grim facial expression, saying, “What are you doing?” and “You'd better not go up there.” That night, however, Ryuunosuke's curiosity drives him to go upstairs after all...

(Source: Natalie, translated)