This creature is a demon who came to the human world as a result of a ritual, which was performed earlier by the woman’s samurai father.
The ritual involved him writing something on the blade of a sword with his own blood.
Later he tried to remove the markings but he couldn’t.
He did it because their village was in some sort of danger and he hoped to have the demon fight on their side.
The movie is set in the modern era and we get to see the demon as a very old man.
The demon-man has a huge spider-like shape under the skin of his forehead and it can reincarnate itself by impregnating women in their dreams.
It’s presence within the dream is indicated by a whirlpool which appears in whatever liquid happens to be present in that dream.
In one dream, the whirpool appears in a cup of coffee, which is on a bedside table, while the demon is having intercourse with a woman in the bed.
