Helen Grant's "Bonschariant," or the "glass demon" of the book, is based on a German legend about Count Sigebodo who had a foreign servant whose name was so difficult to pronounce that the Count called him “Bonschariant,” meaning “the good servant.”
The story follows:
The rich Count Sibodo lived in the time of the Emperor Henry I, and was but a lukewarm Christian. One day the devil appeared to him in the shape of a servant, calling himself Bonschariant. The count took him to his castle on the River Ahr, where the new servant carried out all his orders so skillfully, that eventually his master could not do without him. When the knight went out to a battle, or to a tournament, Bonschariant always had to go with him. Sibodo also took him into the Holy Land, and wherever the servant fought at the side of his master, the latter was always victorious.
When Sibodo came back from the East he was engaged in fierce battles around the Rhine. In one he had managed to beat his enemy, and to force him back across the river. One evening, weary from fighting, he fell asleep under a tree. His enemies crept up and surrounded him, and would have killed him. But Bonschariant rushed up just in time, snatched the sleeping man from their midst, and flew up into the air with him. Sibodo woke up as they were floating through the air, and cried out in alarm, "God have mercy on me." The devil was angry when he heard these words: he snarled and made a dreadful noise. From that day Sibodo regarded his strange servant with mistrust and secret anxiety.
After some years the wife of Sibodo fell very ill. The knight called in the most famous doctors from far and wide, but they could not cure the countess. At last one of the doctors said, "There remains but one way of curing the sick woman, lion's milk and dragon's blood. But who could obtain them?" "I will", said Bonschariant, and flew off through the air towards the south. Two hours later he came back from the heart of Africa with the strange medicine. The countess recovered and was in better health than ever. But when she heard how she had been cured, she begged her husband to dismiss his servant for she was a devout woman, and was sure he must be the devil himself.
It was hard for Sibodo to do without Bonschariant, but to pacify his wife he decided to build a monastery amongst the Eifel hills and to call it Steinfeld. He told his servant that the fine building was to be a hunting lodge. When the work was nearly completed, the count had a cross set up during the night on the very top of it. Early the next day the devil saw the cross as he was dragging a heavy stone along. He began to rage and fume and he cast the stone at it, but the missile was turned aside by an invisible hand and fell far away near Dieffenbach, where it still lies today. From that time on Bonschariant was never seen again.