Riddle 91: Key

My head is forged by a hammer, wounded with pointed tools, rubbed by the file. Often I gape at what is fixed opposite to me, when, girded with rings, I must needs thrust stoutly against the hard bolt; pierced from behind I must shove forward that which guards the joy of my lord's mind at midnight. At times I drag my nose, the guardian of the treasure, backwards, when my lord desires to take the stores of those whom at his will he commanded to be driven out of life by murderous power.