The Flying Serpent

Quetzalcoatl will kill anyone who possesses its feather because the creature views the feather as something uniquely its own that must be recovered. When Forbes plucks the feather from the imprisoned beast, he explicitly notes that the creature will kill to reclaim what is uniquely theirs. The serpent tracks down whoever holds the feather and murders them before returning to its cave.

Forbes taunts the serpent about the feather early in the film, establishing that the creature's rage is tied to this artifact.

Forbes returns to the cave where Quetzalcoatl is imprisoned and releases the creature from behind its bars. Once freed, the beast leaves its lair in search of whoever possesses its missing feather, kills them, then retreats back to the cave. Forbes uses this method twice, first to kill Dr. Lambert, then to target Thorpe and the investigators.

The opening scroll establishes that Forbes has kept Quetzalcoatl imprisoned for five years, giving him the power to release or confine the creature at will.

Forbes's late wife died under circumstances that resemble Lambert's death, both victims were killed by Quetzalcoatl after coming into possession of one of the creature's feathers. Forbes gave his wife a feather, which drew the beast to her. This earlier murder established the pattern that Thorpe later notices during his investigation, raising suspicion about Forbes's involvement.

Forbes mentions plucking a feather from the beast and hints that the creature will kill to recover it, suggesting he knew the danger from the start.

Forbes staged an encounter to plant the feather where Lambert would find it. Lambert had recently written about Aztec carvings of Quetzalcoatl, making him a plausible candidate to discover and keep such an artifact. Forbes then released Quetzalcoatl to track down and kill Lambert, using the feather as a deadly lure to eliminate someone he viewed as an enemy or obstacle.

Forbes deliberately drops the feather near Lambert, showing the murder was premeditated.

When Thorpe is assigned to investigate Lambert's death, Forbes plans to plant the feather on Thorpe to implicate him with the murder weapon. This would derail the investigation and direct suspicion away from Forbes himself. The scheme demonstrates Forbes's willingness to manipulate the authorities to protect himself.

Forbes resolves to plant the feather on Thorpe after Thorpe begins connecting clues to Forbes.

Sheriff Hayes obtains the feather that Forbes planted during his scheme to frame Thorpe. Since Hayes is now in possession of the feather, Quetzalcoatl hunts him down and kills him, just as it did with Lambert. The feather remains a deadly object that draws the monster to whoever holds it.

Hayes spots and seizes the feather before Forbes's plan can fully unfold, but possession of the feather is what seals his fate.

Thorope connects the dots by noting that Forbes's late wife died under circumstances similar to Lambert's, he realizes both victims were killed by the same unknown method. This pattern points to Forbes as the common link, suggesting he orchestrated both murders using the same deadly mechanism.

The investigation reveals the similarity between the two deaths, giving Thorpe the key insight.

In the climax, Thorpe and Hastings spot Quetzalcoatl as it rises and prepares to fly away from the area around the cave. The beast escapes, leaving the investigators on its trail. The ending implies the threat is identified but not yet eliminated, with the serpent still looming as a danger.

The creature has consistently returned to its cave after each killing, but this time it flies away, suggesting a final escape.