This final canto is the climax of the Inferno, the meeting with Satan. The sinners in this final round, Judecca (named after Judas Iscariot), keeping with the theme of retribution, are permanently frozen in the ice; they were treacherous to their masters, the ultimate sin of malice, and are forever encased in their sin of coldness.
Dante's two-fold theme of religion and politics is found in the very mouths of Satan. The ultimate sinners of this kind of malice spend eternity being chewed and flayed by Satan's teeth. The greatest sinner of the world is Judas Iscariot, the man who betrayed Jesus with a kiss. Both Brutus and Cassius betrayed Caesar, founder of Dante's beloved Roman Empire.
The image of Satan is a startling one, beginning with its three faces, which symbolize the perversion of the Holy Trinity. Dante says that Satan is as ugly as he was once beautiful, recalling his former incarnation as an angel. Satan, here, seems less powerful than traditionally depicted; he is dumb and roaring, trapped in the ice, punished as the rest of the sinners, perhaps worse.
