Lamia is a monster which is most often encountered at night in deserts on the southern part of the continent. These creatures are resistant to iron and regarded by some as the variety of vampire. They have the ability polymorph or shapeshift and can transform themselves into beautiful "women". Perhaps "woman" is not the most accurate description as they appear as large snakes with female human heads with bloody faces and fangs.
They are mentioned in the same breath as giant scorpions, frighteners, wights, scarletias and crab spiders. Their fearsome reputation has resulted in a weapon, a very nasty whip, being named for them.
In the PnP RPG
According to the Wiedźmin: Gra Wyobraźni RPG, lamia are described very similarly, but unlike the novels, they take the complete form of a woman and lure their victims into traps.