Tartarus

Tartarus is the great pit beneath the earth used as a dungeon of suffering and agony and a burial place for the monsters, the Titans, and in later myths, for mortals who perpetrated unpardonable sins. It was also the lowest part of the universe, a separate entity lower than the Underworld. 

Some described it as a vast depth, both dreary and damp, and a place of wreckage. The Underworld was the final resting place of mortal sinners, while Tartarus was used for banished monsters and gods alike.

"So how do we get there? Through the river Acheron?" Zion asked.

Azalea nodded.

"There were seven rivers of the Underworld that connected and ran through Tartarus. Acheron was the fastest way from Terra to the Underworld then to Tartarus." Azalea explained.

Amaris was amazed by Azalea's knowledge of things. At first glance, she looked like an innocent girl that didn't know anything, but once you talked to her, she had a vast knowledge about the world.