Cairn no Kubi-tachi
English name: The Spirits of Cairn
Genres: Avant Garde, Award Winning
Type: Movie, Episodes: 1
Aired: 2018
Source: Original
Rating: PG-13 - Teens 13 or older
Synopsis:
The Spirits of Cairn imagines a state of limbo which is the destination of those who die in childhood. The protagonist feels remorse about having died too young, but the emotion is just an echo and he has no idea how long he's been there, why he is there, or the source of his regret. His work of interring the many necks buried below the surface continues without end. 

(Source: Official Site)