Cult of the Mothman
During the Great War, the Cult of the Mothman took shelter in the mine after Charles, a cult member, led them there. However, some members "turned to darker worship" which splintered the cult between worshipping the Wise Mothman and the Interloper. Those who remained worshipping the purple-eyed mothman became known as The Enlightened, and everyone else, the Dim Ones to them. The Enlightened would write about the Interloper in the tome False Gods of Appalachia, stating that the faithful should not speak of it, and that, "there is no fouler deceit than one garbed in truth." They are instructed to, "shun its call, for it can bring only darkness."
As the cultists needed hydration to survive, according to the series of notes spread throughout the mine, supernatural events occurred as the cultists inhabited the location, including the formation of a water spring. The Interloper worshipers mentioned blood "wept" from the being's branches, and that the being taught the cult to share. The being supposedly also told its worshipers that they would be denied into the mine three times, and the being communicated through a woman known as the First Priestess of the Wood. Terminal entries mention a strange old woman being denied to the mine before the Great War, although it is unclear if she is the priestess.
A sect from the Cult of the Mothman, the Dim Ones, still congregate within Lucky Hole Mine and continue their zealous worship. They kneel before the Interloper with arms raised high, offering human sacrifice and blood-filled jars to the Interloper.
A mothman cultist, Jeff Lane, mentions the Interloper in recorded holotape messages. In the messages, Lane describes the nature of the Interloper's "calling" to himself. While previously just a "false memory," the creature's existence was confirmed to Jeff Lane after hearing a tale of the entity "made real" deep underground by a mad storyteller. It is at this point at which Lane describes the Mothman to be merely a creature, more similar to the human race than the Interloper and the "unknowable horrors" hidden within the subconscious mind.
The creature later calls to Lane in a lucid vision, inviting him deep into the depths of the Lucky Hole Mine. He explains how that although it appears others have also heard the Interloper's call, the entity summons him alone in the end. Lane further states that the Interloper chose him as the "conduit of the unknowable" and how the "hidden reality" will finally become revealed.
The Interloper is a large dormant creature found in the depths of the Lucky Hole Mine. It has several appendages protruding from its head and back. The creature will bleed if struck by the player character and its tentacles occasionally move. An audible hum is present heard near its body, which is surrounded by fog, skeletons, corpses, and effigies.
The Interloper is a static object, and does not have any interaction with the player character.
The Interloper appears only in Fallout 76.
Behind the scenes
The game files define the creature as an actor mesh and designate it "gutpuker."