Seven Misaki

Seven Misaki, or Shichi-Nin Misaki in Japanese, is a name given to a group of seven vengeful spirits.
They are spirits of seven people who had met their ends in tragic ways, such as by drowning. They usually haunt places like sea or riverside.
The Seven Misaki's souls are so full of grudge that they will not rest until they succeed at possessing and eventually killing another seven human beings: only then they become appeased and are able to leave this world.
The unfortunate victims of Seven Misaki become the new Seven Misaki. And they too, must look for another seven victims to replace them, so that they can rest in peace.