Despite the fact that the origin of mysterious entities remains unclear to humans, one particular type does not fall into this category.
This special type of entities has a straightforward name: "Revenant."
As the name implies, a Revenant is a mysterious entity that has transformed from the dead.
Their deaths are often accompanied by extremely strong emotions: fanatical obsessions, distorted feelings, deep-seated resentment... Not every person who dies with these emotions will become a Revenant, but every Revenant transformed from the dead undoubtedly carries emotions of this kind.
"Revenants" differ from what is narrowly defined as ghosts; they are not purely Spiritual Body Type Mysterious Creatures but are memetic entities carrying the deceased's emotions and fragments of memories, which possess (or infect) certain things—these things could be the deceased's own corpse, other humans, animals, plants, objects, or even certain phenomena.
