Polterghast

"Long-dead prisoners seek their zealous revenge..."

Polterghast is a post-Moon Lord boss that can be fought in the Dungeon. While it is optional, it is highly recommended to fight it before challenging The Devourer of Gods.

Defeating Polterghast will provide the player with Ruinous Souls, a material used to craft powerful equipment. Additionally, mini bosses in the Abyss will begin dropping weapons upon its defeat, and the final tier of the Acid Rain event will become unlocked.

Summoning
Polterghast can be summoned by using a Necroplasmic Beacon in the Dungeon. Alternatively, if it has not yet been defeated in the world, it will spawn after killing 30 Phantom Spirits, which can be found in the Dungeon after Moon Lord has been defeated. After defeating it once, using a Necroplasmic Beacon becomes the only way to summon the boss.

Behavior
Polterghast behaves similarly to Plantera, chasing the player quickly while using four long appendages to grapple blocks. In the second phase, however, these appendages will detach and begin firing projectiles independently, before re-attaching in the final phase as Polterghast summons an unbound clone of itself.

Polterghast will receive a boost to its speed and damage depending on the size of the arena it is fought in, which is determined by the amount of active tiles in an area around it. If the arena is smaller it will move slower and not be given damage boost, and if the arena is larger it will move faster and be given a damage boost, both proportional to the lack of blocks. Its projectiles' speed and rate of fire are also affected by these boosts.

If the player travels to the Surface outside of the Dungeon, Polterghast will enrage, gaining a speed boost until they return. Additionally, while enraged, Polterghast will gain double damage and defense. If they remain on the surface for over 10 seconds, Polterghast will accelerate away and despawn.

Phase 1 (Polterghast)

 * Polterghast follows a Plantera-like behavior, placing its hooks on nearby walls to quickly pull itself towards the player.
 * Polterghast alternates between moving slowly while firing a spread of 5 slow, bouncing Phantom Shots and moving more quickly while firing a spread of 4 fast, homing Phantom Blasts that move through blocks. These are cyan projectiles and they inflict the Nightwither debuff for 3 seconds.
 * Polterghast's bouncing projectiles accelerate over time until they hit a block.
 * Polterghast periodically stops shooting projectiles and glows red, positioning itself diagonally to the player before charging at them once.
 * Transforms into Necroghast at health.

Phase 2 (Necroghast)

 * Necroghast's hooks detach from their chains, and they begin independently moving and shooting ghostly projectiles at the player.
 * Necroghast moves more quickly and deals more melee damage.
 * Necroghast begins firing a spread of 5 Potent Phantom Shots or Blasts, which are red projectiles that travel faster, further, and deal slightly more damage. They inflict the Whispering Death debuff for 3 seconds.
 * Necroghast charges a second time while it is glowing red.
 * Transforms into Necroplasm at health.

Phase 3 (Necroplasm)

 * Necroplasm's hooks move slower and less often. The chains reappear and the hooks no longer shoot projectiles.
 * Necroplasm moves significantly faster and deals even more contact damage. Additionally, it fires a spread of 6 Shots every 7 seconds.
 * Necroplasm summons an invincible clone of itself. The clone behaves similar to Necroplasm, staying within the hooks. It alternates with Necroplasm in chasing the player.
 * While glowing, the clone positions itself on the other side of Necroplasm and charges up to 6 times, while Necroplasm charges 3 times.
 * When below health, Necroplasm begins summoning up to 2 Happy Phantom Spirits, which also fire ghostly projectiles at the player.
 * When below health, Necroplasm begins to move more quickly as a last-ditch effort to attack the player, and will also stop summoning Happy Phantom Spirits.

In Expert Mode

 * Polterghast's velocity and acceleration are greater in all three phases.
 * The rate Polterghast fires Phantom Shots and Phantom Blasts is increased.
 * This includes their red versions in later phases.
 * Polterghast's hooks can travel further between latches.
 * In its third phase, Necroplasm summons three invincible Phantoms which slowly revolve around itself. The Phantoms fire phantom orb mines which are initially slow, but quickly accelerate toward the player.
 * The phantom orb mines will not deal damage until they reach maximum velocity.

In Revengeance Mode

 * Polterghast's velocity and acceleration are even greater in all three phases.
 * Transitions into Phase 3 (Necroplasm) at 50% health instead of 33%.
 * Spawns Happy Phantom Spirits at 33% health instead of 20%.
 * Moves much faster at 10% health instead of 5% health.
 * Phantoms shoot more often.

In Death Mode

 * Transitions into Phase 2 (Necroghast) at 90% health instead of 80%.
 * Transitions into Phase 3 (Necroplasm) at 60% health instead of 50%.
 * Moves much faster at 25% health instead of 10% health.
 * Phantoms Spirits shoot at an even higher frequency.
 * Gains velocity and acceleration as HP decreases.
 * Can hook increased distances as HP decreases.
 * Hooks faster and more often as HP decreases.

Aftermath

 * After you defeat Polterghast, a message will say, "The abyssal spirits have been disturbed."
 * New content in the Sulphurous Sea and the Abyss will become unlocked:
 * The third tier of the Acid Rain and all of its associated content will become available.
 * The Old Duke boss becomes accessible, spawning at the end of the third-tier Acid Rain event.
 * Bloodworms also begin spawning in the Sulphurous Sea, which can be used when fishing to summon The Old Duke.
 * The Reaper Shark will begin dropping Reaper Teeth, the Valediction, Deep Sea Dumbbell, and The Reaper.
 * The Juvenile Eidolon Wyrm will begin dropping the Eidolic Wail and the Soul Edge, and the Adult Eidolon Wyrm will begin to spawn naturally.
 * The Colossal Squid will begin dropping Calamari's Lament.
 * The Mauler will begin dropping the Sulphuric Acid Cannon.
 * The Bobbit Worm will begin dropping the Bobbit Hook.
 * Dungeon Crates will contain Phantoplasm, and Abyssal Crates will contain Reaper Teeth.
 * The Cyborg will begin selling the Lion Heart.

Tips

 * Polterghast has similar AI to Plantera, which means that movement strategies used during that boss fight may work here as well.
 * Slowing down and observing how Polterghast behaves shortly before each of its telegraphed dashes would help dodging it, as the boss will only dash diagonally from where it's positioning itself relative to the player.
 * For phase 3 especially, dodging towards an axis parallel to where Polterghast and its clone line up would make dodging both a lot easier.
 * Don't rush too much when fighting its last form. The chaotic activity can easily overwhelm the player if they're careless. Take your time and be careful when dodging.

Trivia

 * Polterghast's theme is Feral Amalgamation, which was made by the artist DM DOKURO.
 * If the Calamity Music add-on mod is disabled, Plantera's theme will play instead.
 * Polterghast’s theme used to be Ruin, a remix of "In the Hall of the Mountain King" from the game The End is Nigh.
 * Polterghast's clone in Phase 3 was inspired by Pontiff Sulyvahn, a boss from the game Dark Souls III.
 * The boss's multiple names are the result of a disagreement between the boss's spriter, LordMetarex, and the lead developer, Fabsol/MountainDrew. The spriter was naming his files ‘Necroplasm’ and was using it as the official name for the boss. During a spoiler, the official name was revealed to be ‘Polterghast’. After an argument the developer decided to use ‘Polterghast’ for the first phase and ‘Necroplasm’ for the last phase. ‘Necroghast’, being a combination of the two names, was used for phase two.
 * In Polterghast's second and third phases, the Boss Health Bar displays the name of the phase followed by "the Polterghast", similarly to how Town NPC names are displayed in Status messages.
 * In the mod's source files, Phantoms are referred to as 'PhantomFuckYou'.