Duke Hellron

Duke Hellron is a Post-Moonlord boss, it is a more advanced form of Duke Fishron and is summoned with a Levaithan Hotline Rod and a Moltipede as bait. It can only be summoned in the Underworld, from a pool of at least 50 lava tiles, with the aforementioned rod and bait, and if both Duke Fishron and the Moon Lord have been defeated on that world. It drops inferno scales upon death, a required item to build the Underlord Armour. It has 3 forms in total, 4 on Expert mode, and changes appearance upon reaching it's third form.

First Form
Duke Hellron will chase the player, spitting lava bubbles or Waspfish at the them. The player will be unable to leave the Underworld, as thick ash clouds will cover the underworld's roof. It will fly in loops or fly incredibly fast past the player to overtake them occasionally.

Second form
(Upon reaching 40,000 health) Duke Hellron will begin to fly faster and will stop spitting waspfish and lava bubbles, instead it will spit burning ash clouds at the player and summon Flamnadoes. It may also spit an arching stream of lava similar to the ichor sticker from time to time.

Third Form
(Upon Reaching 10,000 health) In it's Third Form, Duke Hellron's skin visibly becomes obsidian, this causes it's contact damage to fall to 75, but doubles it's defense. It's lava spitting behaviour becomes more common and it will summon flamnadoes more often, which Waspfish will now spawn from. The Ash Clouds covering the Underworld Roof will now rain lava, inflicting the Melting Armour debuff. The boss itself will charge the player more often, as well as sometimes flying off screen and then suddenly flying at the player rapidly, similar to the Queen Bee.

Fourth form
In Expert mode only after reaching 2000 Health, Duke Hellron will begin to charge the player even more often and it's defense will increase to 120, in addition, the pools of lava within the underworld will begin to spawn lava bubbles that will travel upwards and pop upon contact with the player. Large black ash clouds will also begin to travel around the Underwold, obscuring the players view of the boss.

Defeat
Upon the bosses' defeat, it will spawn a hallowed brick box in which it's drops will be contained. The Angler will begin to offer more advanced fishing quests and the Arms Dealer will begin to sell Infernal Arrows and Scorch Bullets. Several much more dangerous monsters will begin to spawn in the underworld. Molten Hearts now also have a 5% chance to drop from Underworld enemies.

Trivia

 * The boss was originally going to be called Duke Moltron however, the name was taken.
 * The Leviathan Hotline Rod is built only to battle the boss.
 * Using the Magic Mirror or a recall potion to travel home will cause the player to be killed within a few seconds by the now enraged Duke Hellron. The Boss will rapidly fly to the players spawn point and deal enough damage to kill most players outright.
 * It is considered the most difficult of the three Post-Monnlord upgraded bosses.
 * It's Expert mode item is the Hellstone Charm.