Kerberos

The underworld is normally accessed by cave and then tunnel, and generally speaking little of the underworld is protected.  It can be accessed relatively freely, although it is not the most pleasant of places.  But certain portals of the underworld are protected.  The main one commonly recognised in all myth and legend is the one from the underworld to earth in the area in which newly departed souls accumulate.

Kerberos or Cerberus was a three headed dog found in Greek legend and myth.  He was the protector of the underworld and guarded the gates of ‘Hades’. More specifically, he was there to prevent access to specific parts of the underworld.  He is described below in the observations.

Each of Cerberus' three heads is said to have an appetite only for ‘live meat’ and thus allows the spirits of the newly dead to freely enter the underworld, but does not allow the living to enter. 

Most occurrences in ancient literature describe ways in which Cerberus can be overcome, for example, in the Aeneid Cerberus is lulled to sleep after being tricked into eating drugged honeycakes, and Orpheus puts the creature to sleep with his music.

Observations

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• Cerberus, watercolour by William Blake
• Kerberos - As described in Ovid's Metarmorphosis
• Kerberos according to Virgil
• Kerberos and Hercules