Nightwraith

When the sun sets, the fields become the domain of midnight demons who draw their power from moonlight. Dark as the night itself and filled with grief, they roam the fields lit by silver light. Woe to those who are spotted by a nightwraith.

From the original Polish, it is unclear if these creatures ("Północnica") are the same as the "nocnica" which are described as creatures who appear on the eve of Saovine as seemingly friendly women, possibly even midwives, but they curse pregnant women causing them to give birth to strigas.

"Nightwraiths are born of moonlight, wind and the earth cooling after the heat of the day. They rise above the ground and whirl in a mad dance, which should not be seen by any mortal. If caught peeping, the mortal is blinded by moonlight, then taken into the circle and forced to dance until he expires, at times becoming a nightwraith himself."

Location

 * Fields

Source

 * Specters, Wraiths, and the Damned
 * Geralt can talk to an "Old peasant" around Murky Waters who will mention terrible dreams about Nightwraiths. The conversation will not result in a bestiary entry for these monsters though.

Gallery
Půlnočnice Nachterscheinungen Aparición nocturna Spectre de minuit Déllidérc Wraith notturna Nakties šmėkla Północnica Полуночница