Here is Heinrich Heine's description of Wilis. This has been translated by Marian Smith and is included in her new book, Ballet and Opera in the Age of Giselle (Princeton, 200O). This description was also printed at the beginning of the Giselle libretto in 1841:
GERMAN TRADITION
From which the plot of the ballet Giselle or The Wilis is taken.
There exists a tradition of the night-dancer, who is known, in Slavic countries, under the name Wili. -- Wilis are young brides-to-be who die before their wedding day. The poor young creatures cannot rest peacefully in their graves. In their stilled hearts and lifeless feet, there remains a love for dancing which they were unable to satisfy during their lifetimes. At midnight they rise out of their graves, gather together in troupes on the roadside, and woe be unto the young man who comes across them! He is forced to dance with them until he dies.
Dressed in their wedding gowns, with wreathes of flowers on their heads and glittering rings on their fingers, the Wilis dance in the moonlight like Elves [italicized]. Their faces, though white as snow, have the beauty of youth. They laugh with a joy so hideous, they call you so seductively, they have an air of such sweet promise, that these dead bacchantes [italicized] are irresistible.
Heinrich HEINE (On Germany) [italicized]