Dr. Gruener was a Jewish man who was placed in a concentration camp during World War Two. Many years later, Dr. Gruener took up the identity of The Reaper to kill Colonel Kurt Schloss, who was the former head of the concentration camp he was imprisoned in. Though Gruener successfully killed Scloss during a halloween parade, he ran afoul of Batman and apparantly died during their battle. It turned out many years later he was saved and cryogenically frozen by Black Mask. He has recently been thawed out.