The Old Duke

"The mutant terror of the seas was not alone..."

The Old Duke is a post-Moon Lord boss that can be fought in the Sulphurous Sea biome after defeating Polterghast.

Spawning
The Old Duke can be spawned by finishing the Acid Rain event after Polterghast is defeated, appearing from a giant sulphurous tornado. It can also be summoned by fishing in the Sulphurous Sea with a Bloodworm.

Behavior
The Old Duke behaves similarly to Duke Fishron at its core, though most of its attacks are entirely unique and it features distinct differences in behavior. When the Old Duke charges at the player, the direction it charges takes the player's velocity at the time the charge began into account, aiming toward the player's predicted destination and potentially catching them off-guard. Upon contact, the player is inflicted with the Venom, Poisoned, Irradiated, and Feral Bite debuffs for 5 seconds.

The Old Duke is normally very resilient, with high defense and DR. However, every 10 attacks, it runs out of stamina and becomes exhausted, pausing for 5 seconds before resuming its attacks. While exhausted, the Old Duke's defense and DR are reduced to zero, leaving it vulnerable to attack.

If the player exits the Sulphurous Sea, the Old Duke charges at a much greater speed and distance.

Phase 1
The Old Duke's attacks function on a set cycle, being performed in this repeating order:
 * Charges three times with short breaks in between, with the charge direction accounting for player's velocity at the time the charge began.
 * Spits five invulnerable Tooth Balls which target the player for 8 seconds, before turning themselves vulnerable and stationary after 10 seconds.
 * Killing the Tooth Ball, contacting it, or letting it live for 15 seconds will cause the Tooth Ball to erupt into 4 and 8 homing teeth outward in a cross formation, along with a sulphur cloud at the center of the explosion.
 * Charges three more times, again accounting for the player's velocity.
 * Pulses, and fires 2 groups of 5 vulnerable Sulphurous Sharkrons, one group from the left and the other from right.
 * The Sulphurous Sharkrons inflict the Venom, Poisoned, Irradiated, and Feral Bite debuffs for 3 seconds on contact.
 * The Sulphurous Sharkrons explode into 2 damaging chunks when killed or upon tile contact. Sulphurous Gores inflict the Poisoned and Irradiated debuffs for 3 seconds.

Phase 2
Transitions at health, becoming briefly invulnerable and firing 6 Sulphurous Sharkrons upward as it does so. Its charges and movement speed are faster, and its attack cycle is now modified:
 * Charges twice.
 * Flies in quick loops, spitting 5 Tooth Balls and forming a massive circular which draws the player into and around it. The vortex lingers for a significant amount of time after this attack.
 * Charges twice again.
 * Summons 2 groups of 5 Sulphurous Sharkrons along with vomiting a large amount of gravity-affected Sulphurous Gore chunks.
 * Gains a yellow glow in its eyes and some parts of his body.

Phase 3
Transitions at health. The Old Duke will become more translucent and green, temporarily gaining invincibility for its first two attacks. After it teleports the first time, it becomes vulnerable again.

Its attack cycle during Phase 3 is the same as Phase 2, though its charges and movement speed are even faster. It also gains the ability to teleport to a location nearby the player before it starts its charge series. The teleports do not count towards the Old Duke losing stamina. Additionally, the Old Duke charges three times before summoning the Sulphurous Vortex instead of twice.

In Revengeance Mode

 * Switches between its attacks faster and has increased movement speed.
 * Charges are faster in velocity and occur more frequently.

In Death Mode

 * Switches between its attacks even faster and has further increased movement speed.
 * Charges are even faster in velocity and occur even more frequently.

Resistances
The Old Duke takes decreased damage from some items and projectiles. It takes:
 * 50% damage from the ink fired by the Calamari's Lament minions.
 * 55% damage from the Crescent Moon's flail.
 * 60% damage from the Crescent Moon's projectile and the explosions from the Bloodflare armor's ranged blood bombs.
 * 75% damage from the Soul Edge's souls and Ethereal Subjugator's minions.
 * 80% damage from the explosions of the Lunar Flare, Stellar Striker, Stellar Contempt and the green Mad Alchemist's Cocktail Glove cocktails.

Aftermath

 * The Sea King will start selling Bloodworms.

Tips

 * Although the Old Duke is vulnerable during its attacks, it takes greatly reduced damage compared to the damage it takes while it is exhausted. Burst damage options such as Adrenaline and Rage in Revengeance Mode should be saved for when it runs out of stamina.
 * If the Old Duke is off-screen, listen for the distinct "huffing" sounds it makes while it is exhausted.
 * Dashing after the Old Duke's charges can help significantly with dodging them.

Trivia

 * The theme that plays during the Old Duke's battle is Toxic Wisdom, which was composed by the artist DM DOKURO.
 * If the Calamity Music add-on mod is disabled, Boss 1 will play instead.
 * The Old Duke was inspired by the Old King from Enter the Gungeon.
 * The Old Duke is sometimes referred to as "Boomer Duke" in the mod's files, referencing the meme in which older people are referred to as "boomers".
 * The tornado which appears when The Old Duke is summoned at the end of the Acid Rain is referred to as 'OverlyDramaticDukeSummoner' in the mod's files.
 * If the player attempts to summon the Old Duke using a multi-lined hook, such as the Early Bloom Rod, it will lead to multiple Old Dukes spawning in at once.
 * This is an homage to an oversight in vanilla Terraria, where multiple Duke Fishrons can be summoned using a fishing exploit.
 * The Old Duke is currently the only boss without a lore entry on the mod's official forum page.
 * The Old Duke was released on April 1st (April Fools Day) because Duke Fishron was also released on that day.