P1 AI
Polterghast is a post-Moon Lord boss. Defeating it unlocks a large selection of content, including the final tier of the Acid Rain, weapon drops from major Abyss enemies, and items crafted with Ruinous Souls.
If Polterghast has not yet been defeated in the world, it will spawn after killing 30 Phantom Spirits, forewarned by the status messages "Wails echo through the dilapidated dungeon halls..." at 10 kills and "Long-dead prisoners seek their zealous revenge..." at 20 kills.
Polterghast can also be summoned by using a Necroplasmic Beacon in the Dungeon. This becomes the only way to summon the boss after defeating it for the first time, as it will no longer spawn via Phantom Spirits.