Volcanox

Volcanox is a post-Moon Lord boss that can be fought after defeating Moon Lord. Defeating Volcanox will provide the player with Pyroplasm, Volcanic Stone and one of 5 weapons.

Spawn

 * Volcanox can be summoned with the Charred Core anywhere in The Underworld at anytime.

First Phase

 * He behaves similarly to Plantera, chasing the player while hooking on nearby blocks.
 * He spawns Fireflies which chase after the player.
 * He shoots fireballs exploding into Greek Fire, those become faster as Volcanox loses HP.

Second Phase

 * Once he is brought down to the half of his max HP, he changes into his second form - his outer shell falls off, revealing a fiery body.
 * Volcanic Tentacles sprout from it, which try to attack the player.
 * He spawns a Soul of Infernace to aid him.
 * Volcanox shoots a barrage of fireballs, much like his first phase. Sometimes, those get enlarged and deal more damage.
 * He also releases a ring of exploding fireballs sometimes.

Trivia

 * Volcanox's Theme is "Inner Core", which was composed by Ranipla.
 * Him spawning the Soul of Infernace is a reference to his lore, as he was the one who granted Infernace those powers.

Lore
Volcanox is the primordial manifestation of fire, ash, and magma. Coalesced from the immense thermal energy of the Underworld itself, his power far exceeds any mortal being and is easily comparable to the divine beings that shape the universe of Terraria, some of them even fearing him. He is often referred to as the heart of the Underworld itself, for his power alone retains the heat within the depths. He is responsible for many of the horrors within the underworld, such as the Wall of Flesh, and of course Infernace, his first and arguably most significant creation. His magnanimous contributions to the Underworld sparked an ancient civilization, now left in shambles due to the events that transpired.

On one fateful day, a great ancient being of terror materialized within the dimension of Terraria and the Underworld was torn asunder by its unholy force. Volcanox was forced to take action as the terrible force sought to blot out the heat and life of the Underworld. As the entity known as the Void Leviathan was an incomprehensibly powerful beast which warped reality itself, Volcanox could not take this lightly and he unleashed a superheated force that made Terraria itself quake. The godly entity of the void clashed valorously with Volcanox, but was unsuccessful and was forced to slink out of the Underworld, his conquest attempt failing.

Volcanox did not win the fight with surprising ease. Battered and drained from the demanding force required in battle, he sunk into the core of the Underworld, in a place far hotter than any mortal could survive, not to be trifled with until the defeat of his grand creations, Infernace and the Wall of Flesh. It was only after the defeat after the great lord of the moon that Volcanox sought to crush the Terrarian before they gained too much power and threatened the peace of the Underworld itself.

But he was far too late to act...