Who is the sad eyed angel? Why does eliezer refer to the three gallows that are going to be used to murder the hound pipel "three black Ravens"?

Eliezer saw many executions, and the victims, having already lost their capacity for emotion, never cried. Only once did the jaded, dried-up prisoners weep at an execution. An Oberkapo and his pipel, the sad eyed angel, (a young boy who acted as his assistant) who everyone liked were suspected of blowing up a power plant on camp, but they refused, despite torture, to give any information about it. The little boy, who had the face of a sad angel, was sentenced to be hanged. The "three black ravens" represent the hanging of the three prisoners.