In folklore, a succubus (plural succubi) is a demon who takes the form of a highly attractive woman to seduce men (although the attractive part is a modern invention, historically succubi were ugly), in dreams to have sexual intercourse, according to the medieval European legend. Its male counterpart is the incubus. They draw energy from the men to sustain themselves, often until the point of exhaustion or death of the victim.
In The Witcher saga, Regis had a romantic relationship with a succubus while he, the witcher and their companions were in Beauclair.
The torched village near Vergen proved to be the home of a real succubus. This beautiful creature, possessing hooves instead of feet, drew handsome young men to it in order to seduce them and feed on their life forces. The seductress was accused of brutally murdering many young men, and the accusation seemed to be solid. For you must know that the unpleasant scent of sulfur is not the sole inconvenience one must endure when encountering succubi, which can be as dangerous as vampires.
Geralt investigated the murders and concluded that the succubus was indeed guilty of the murders of which it was accused. The witcher had no choice but to slay the dangerous, though intensely beautiful, creature.
Geralt investigated the murders thoroughly. He concluded that the succubus was innocent and that it was being framed by a jealous lover. The witcher cleared the seductress of the charges, and she proceeded to thank him for it in her customary, beautiful manner.