Cerberus, (or Kerberos in Greek), was the three-headed dog
that guarded the Gates of Hell. He allowed any dead to enter,
and likewise prevented them all from ever leaving. He was
bested only twice: once when Orpheus put him to sleep by
playing bewitching music on his lyre, and the other time when
Hercules confronted him and took him to the world of the
living (as his twelfth and last labor).