Der Kindestod

Der Kindeston was a child-killing demon and the chief antagonist of the Buffy episode "Killed By Death" - he was one of Buffy's darkest opponents due to his very nature, which made him arguably amongst the most evil of the series many monsters.

History
Der Kindestod was a demon who killed children by sucking the life out of them, the process left the victims appearing like they died of fever. Therefore it preyed on children who were already ill, leaving little evidence to suspect their deaths were unnatural. One of his victims was a girl named Celia, whose death was witnessed by her cousin, Buffy Summers and traumatised her. Unable to be seen by adults this bogeyman could only be seen by children, the mentally ill or those with intense fever. In 1998, Buffy, now a Slayer, encountered Der Kindestod again while being treated for flu at Sunnydale General Hospital, where the monster was once again committing his terrible acts - Buffy picked up her courage and fought the creature, killing him via snapping his neck.