The name of this article wasn't revealed in the novel and instead was given by editors of the wiki as substitute for lack thereof.
It is a naturally formed coffin that finally came into being. No one can put the thing inside into something else. The thing inside has been there for a very long time and had once drifted throughout the vast three thousand worlds. There are no other treasures in this world that are comparable to it.
A small wooden coffin that was not eye-catching at all. It is around the size of one’s palm and of a purple color. Despite its humble size, it is a delicate work of art. Runes are carved atop of it, but rather than being hand-carved, they feel natural as if they had always been there.