The Spirits of Cairn
This award-winning film has been recognized for excellence in animation and storytelling.
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)
Plot summary from MyAnimeList. Data and images via Jikan API.