Dorocha

The Dorocha or shadow people are the voices of the dead. They appear as demonic, screaming skeletons made of smoke. They can also kill any mortal being they touch, seemingly freezing them to death, and are repelled only by light. In their presence, any magic user loses their powers (actually it's not clear if they can't use magic against the Dorocha or at all). However, even though magic users lose their powers, they are partially resistant to the Dorocha's fatal touch as they do not die instantly like people without magic but die within hours and can survive long enough to be healed later. According to the Cailleach, the Dorocha are numberless.

History
When Morgana tore open the veil between the worlds of the living and the dead, the Dorocha were released into the world of the living, and began launching night attacks on Camelot and villages throughout the land.

The Dorocha were presumably sent back to their world when Lancelot sacrificed himself to repair the veil. (The Darkest Hour)

According to the Cailleach, Morgana's enemies would rue the day the veil was torn, suggesting that if the Dorocha had been allowed to remain in the world of the living, they would have caused massive devastation. (The Darkest Hour)