Zoufu no Hanataba

A horror mystery involving different species. 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 at his grandmother's house, a strange, plant-like thing extends down from above. Intrigued, Ryuunosuke tries to climb the stairs, but his uncle stops him with an unfamiliar expression, saying, "What are you doing?" and "You'd better not go up." That night, however, Ryuunosuke's curiosity drives him to go upstairs after all...

(Source: MangaDex)