The Slavs believed that Mares were the souls of living people who would leave their bodies at night to choke, suck blood, and drain the life force out of other sleeping people. Mares were exclusively female. They were thin, long, and translucent, with big eyes, a thick head of hair, a long thin tongue, and foul breath. They entered the house through any small opening, such as a keyhole, most often under the guise of a small animal or insect, such as a moth, mosquito, bee, wasp, mouse, cat, bird, or frog.
They’d sit on the chest of their victim (people reported crushing feelings in their chests or not being able to breathe) and insert their long thin tongue into the ear or mouth and sucked blood. In the morning, they’d return to their sleeping bodies, while their victim would wake feeling exhausted and drained of energy. The Mare’s victims were most often relatives, friends, and neighbors. Fortunately, Mares rarely brought on death. Nonetheless, they were cumbersome and unwanted night visitors.
The woman who became a Mare after falling asleep usually had no idea. She could be any woman, even one’s wife. The Slavs believed that a seventh daughter became a Mare, or a girl at whose baptism the priest mixed up the words of a prayer or her name, or a girl with very fair hair and a unibrow, or a girl with different colored eyes.
There were many methods to eliminate or prevent Mares. None of them were completely effective though. Beds were turned 180 degrees or moved to another part of the room. Stones with a hole were hung on the bed post. Other methods included turning one’s pillow over and making the sign of the cross, keeping an upside-down broom in the room, or leaving a belt on top of the bedsheets. If the victim woke and found a Mare on their chest, they could tell her “Come in the morning and I’ll give you bread with butter.” She’d disappear and return for payment (as a neighbor) in the morning.
When I was researching this post, I found several discussions on forums about Mares, as in people discussing whether or not they believe in them or discussing being visited by one. Belief in Mares it seems persists to this day among people from rural regions and those who are highly religious. There was one man who believed he was visited by a Mare every time he slept in a room with no blessed objects. Others told of how the Mare disappeared when they said a prayer.
Book: Zych, Paweł and Witold Vargas. Bestiariusz Słowiański. Olszanica: Bosz, 2012. Print.
Fantasy Encyclopedia http://encyklopediafantastyki.pl/index.php?title=Zmora