Inferno Canto XX (the Eighth Circle, Fourth Pouch: Diviners, Astrologers, and Magicians)
After his unprecedented rage at Pope Nicholas III, Dante announces his intent to write material for this canto with a brazen new authority.
When he looks down into the fourth pouch, he sees a group of sinners walking slowly, as if participating in a holy procession.
To his amazement (and our horror), a closer inspection reveals that these sinners’ heads are turned backwards on their shoulders so that they’re forced to walk backwards because they can't see in front of them. And in a sadistically comic twist, they cry while they walk so that their tears trickle down their buttocks.
Dante is so horrified by this that he tells his reader how the tears sprung to his eyes.
As Dante breaks down, Virgil turns on him with scorn. He has no pity for the sinners.
He urges Dante to look at the many sinners here, pointing out famous ones like Amphiaraus (a king who foresaw his own defeat and tried to hide from it), Tiresias (who changed himself from a man to a woman and back again), Aruns (who predicted Caesar’s victory), and the witch Manto (after whom Virgil’s hometown Mantua is named).
Manto is particularly grotesque with her long hair on her backward-turned head covering her bare breasts.
Now Virgil makes a strange sidetrack to verify the origins of Mantua.
He traces the history of Manto, who lived as child in Thebes but left the war-torn town after her father died. She wandered until she came to a marsh around Lake Benaco and there she settled. Later, other people realized the brilliance of having an unassailable—if stinky—marsh town and joined her, founding the town of Mantua.
As Virgil finishes his little tale, he dares Dante to try to discredit him.
But Dante has no interest in doing that and instead appeases Virgil by acknowledging his authority.
As usual, Dante wants to know the names of the sinners in this pouch.
Virgil points out more notable ones, like Calchas (a minor character from his Aeneid), Michael Scot (an astrologer), Guido Bonatti (also an astrologer), and Asdente (a shoemaker/soothsayer).
Virgil decides to move on because the moon is getting low on the horizon, or—as the medieval Italians like to say—"Cain with his thorns already… touches the sea."