Complex sentence: The final part is the Orb so Jack has to find his way to the Necromancers' Land of the Dead through Dismal Swamp and upon reassembling the Staff, he follows Hecubah to the Underworld and defeats her in the final climatic fight.
Split into simpler sentences: The final part is the Orb so Jack has to find his way to the Necromancers' Land of the Dead through Dismal Swamp. Upon reassembling the Staff, Jack follows Hecubah to the Underworld and defeats her in the final climatic fight.