Possible Clues:

• Site of the mythical Lethe River
• Vergil's art first grasped the meaning of the underworld
• The nether world, hell
• Home of five rivers
• Perhaps he'll make the shade
• Seems he'll make it hot for one here
• The underworld where the souls of the dead live
• Pluto's domain
• Greek hell in shade
• The underworld has to keep notes ...