Dante and Virgil climb out of the valley of the thieves and see the next valley below them dark and dotted with small lights like fireflies. As they get closer they see that these lights are really flames with souls encased inside of them. This is the valley of the Deceivers and evil counsellors. Just as in life they sinned with their tongues, they are now condemned to live eternally in tongues of fire. Dante and Virgil approach a flame with two heads holding two souls, Ulysses and Diomede. Ulysses and Diomede planned the plot to use the trojan horse to enter and destroy Troy and subsequently steal the pagan statue of Pallas.
Ulysses (inside the flame with Diomede) then tells Virgil of his last voyage through the Pillars of Hercules (the end of the known world at the Straits of Gibraltar).
Dante and Virgil then move on to talk to another sinner encased in flame.