The Underworld

Deep under the Cavern Layer the underworld begins. The following monsters begin to spawn at around -3100ft regardless of background change: The Fire Imp and the Bone Serpent. Imps shoot fire through walls, big bone serpents pretty much destroy your health, a lot of lava and hellstone.

This is the top tier for now, giving you access to armor, weapons and tools.

Monsters in The Underworld

 * Fire Imp
 * Bone Serpent