A Liderc, or Lidérc, is a unique supernatural being of Hungarian folklore. It has three known varieties, which often borrow traits from one another.
The first, more traditional form of the Lidérc is as a miracle chicken, csodacsirke in Hungarian, which hatches from the first egg of a black hen kept warm under the arm of a human. Some versions of the legend say that an unusually tiny black hen's egg, or any egg at all, may become a Lidérc, or that the egg must be hatched by placing it in a heap of manure. It can assume the form of a chicken or human being, but one leg always has a goose's foot. The Lidérc attaches itself to people to become their lover. If the owner is a woman, the being shifts into a man, but instead of pleasuring the woman, it fondles her, sits on her body, and sometimes sucks her blood, making her weak and sick after a time.
From this source comes a Hungarian word for nightmare, lidércnyomás, which literally means "Lidérc pressure", from the pressure on the body while the being sits on it. Alternate names for the Lidérc are iglic, ihlic in Žitný ostrov, lüdérc, piritusz in the south, and mit-mitke in the east.
It carries out tasks such as finding treasure and thus makes its owner rich, but eventually grows bothersome because it demands work constantly and is always looking ahead to its next task. To dispose of this form of the Lidérc, it must be persuaded to perform an impossible task, such as haul sand with rope, or water with a sieve. It can also be destroyed by locking it into a tree hollow.
The second variety of the Lidérc is as a tiny being, a temporal devil, földi ördög in Hungarian. It has many overlapping qualities with the miracle chicken form, and it may also be obtained from a black hen's egg, but more often it is found accidentally in rags, boxes, glass bottles, or in the pockets of old clothes. A person owning this form of the Lidérc suddenly becomes rich and is capable of extraordinary feats, because the person's soul has supposedly been given to the Lidérc, or even to the Devil.
The third variety is as a Satanic lover, ördögszerető in Hungarian, quite similar to an incubus or succubus. This form of the Lidérc flies at night, appearing as a fiery light, a will o' the wisp, or even as a bird of fire. In the northern regions of Hungary and beyond, it is also known as ludvérc, lucfir. In Transylvania and Moldavia it goes by the names of lidérc, lüdérc, and sometimes ördög, literally, the Devil. While in flight, the Lidérc sprinkles flames. On earth, it can assume a human shape, usually the shape of a much lamented dead relative or lover. Its footprints are that of a horse. The Lidérc enters houses through chimneys or keyholes, brings sickness and doom to its victims. It leaves the house with a splash of flames and dirties the walls. Burning incense and birch branches prevent the Lidérc from entering one's dwelling. In the eastern regions of Hungary and beyond, it is said the Lidérc is impossible to outrun, it haunts cemeteries, and it must disappear at the first crow of a rooster at dawn.
Magyar Néprajzi Lexicon Encyclopedia of Hungarian Folklore (in Hungarian)