In Dante's Inferno, Hell is described in vivid detail in the eyes of Dante, the main character and author. Sinners are eternally punished with tortures that fit their sins. This idea of retributive justice and the role of human reason in the form of Virgil are the two main themes in the poem. Canto VIII contains Dis, the capital of Hell and is most representative of these themes.
The sinners caught in the 5th circle, Styx, are the Wrathful, ones that purposely harm others physically or emotionally. There are tortured by attacking each other with foul slime and tearing at each other's flesh. Just as they had attacked others in life, they are forever being attacked in Hell. In almost every Canto, a new class of sinners and their punishments are introduced. This retributive justice is the most obvious theme of the poem.
During his journey, Dante is guided by Virgil, the symbol of human reason. When they approach the boatman Phlegyas, he becomes enraged that they are not sinners, but Virgil's word convinces Phlegyas to take them across, symbolic that human reason can shine through obstacles. However, Dante address the idea of Hell to be too much for the simple human mind to understand, so an angel must open the doors of Dis for them to pass through.
Canto VIII is the most important Canto because it proves the power of human reason and delves into the retributive punishment of the sinners. Dis, being the capital of Hell, signifies the end of understanding and the beginning of the eternal torture behind the human mind.