Inferno Canto XII (the Seventh Circle, First Ring: the Violent against their Neighbors)
Our two heroes descend down a bank so steep that Dante compares it to the famous landslides of Marco.
At the bottom, they catch sight of the hideous half-man, half-bull creature that is the Minotaur. It responds to the sight of them by biting itself.
Virgil shouts to the Minotaur that Dante is not his hated enemy and slayer, Theseus, the "Duke of Athens," but is only here to watch him suffer. That makes the man-bull furious and he charges them.
Dante and Virgil hightail it out of there, down the embankment.
Once they’re no longer in danger of being impaled by longhorns, Virgil tells Dante how the landslide came about:
When Virgil visited this part of Hell before (on his Erichtho run), he saw Christ rapture the good men from the Old Testament out of Hell. At that moment, "I [Virgil] thought the universe felt love" and it was that primal force that caused the earthquake and created the rubble strewn path.
But enough of Virgil’s past; he now fixes Dante’s attention on the upcoming river which marks the boundary of the seventh circle. It is Phlegethon, full of boiling blood.
But here comes trouble. Along the banks of the fiery river race herds of centaurs, who are half-men, half-horse creatures all armed with bows and arrows.
They surround our pilgrims, threatening them with arrows, but Virgil speaks out boldly, requesting to see Chiron (the head centaur) and refusing to speak to anyone else.
As the two parties cautiously approach one another, Virgil whispers various centaurs’ names to Dante, introducing them and their crimes of violence.
Chiron approaches with an arrow drawn and says to his herd, "Have you noticed / how he who walks behind moves what he touches?" In other words, Dante actually affects the environment when he passes through it… which means he’s alive.
So Virgil brazenly confirms Chiron’s suspicions, telling him that both Dante and he are here on a mission from God. He reassures the centaurs that they are not robbers and asks them to lend him one of their herd to guide them across the river. Oh, and to allow Dante to ride on his back.
Chiron wheels about and chooses Nessus to guide and defend the two poets.
By the way, Nessus’s past crimes including raping Deianira (Hercules’s wife) and indirectly causing Hercules’s death. So our heroes entrust themselves to a half-equine rapist and murderer. Welcome to Hell.
Walking along the riverbank, Dante witnesses the sinners screaming in the boiling blood. Nessus adds his commentary, naming the sinners: Alexander, Dionysus, Ezzolino, Obizzo.
At this point, Virgil concedes authority to Nessus and tells Dante to listen to the centaur as they travel.
Further along, they come across some sinners immersed up to their throats in hot blood. Nessus names one as Guy de Montfort (who murdered Prince Henry).
Dante notices that the depth of the bloody river changes in response to the nature of each sinner’s violent deed.
At a place where the river is only ankle-deep, they cross.
Nessus explains that on this side, the river eventually grows deeper and deeper until it completely submerges the tyrants like Attila, Pyrrhus, Sextus, Rinier of Corneto, and Rinier Pazzo.
Then Nessus leaves them, turning around to cross the river again and go home.