Tartarus was the place of everlasting darkness and was found in the Underworld. It was home to Campe's lair and other unnatural creatures. It is where the most evil people are sent for an eternity of torment.
Jason and Hercules travelled here in order to find Pandora's Box, in order to return it to Kyros, and were joined by the dead Cyrus. There they fought Campe and her undead minions who tried to attack. They both struggled to escape after blowing the horn. (Pandora's Box)