The Village

In the 19th century, residents of the small, isolated Pennsylvania village of Covington live in fear of nameless creatures in the surrounding woods and have constructed a large barrier of oil lanterns and watch towers that are constantly manned to keep watch. After the funeral of a seven-year-old boy, Lucius Hunt (Joaquin Phoenix) asks the village elders for permission to pass through the woods to get medical supplies from neighboring towns; however, his request is denied. Later, his mother Alice (Sigourney Weaver) admonishes him for wanting to visit the neighboring towns, which the villagers describe as wicked. The Elders also appear to have secrets of their own and keep physical mementos hidden in black boxes, the contents of which are reminders of the evil and tragedy they left behind when they left the towns. After Lucius makes a short venture into the woods, the creatures leave warnings in the form of splashes of red paint on all the villagers' doors.
Meanwhile, Ivy Elizabeth Walker (Bryce Dallas Howard)âthe blind daughter of the chief Elder, Edward Walker (William Hurt)âinforms Lucius that she has strong feelings for him, and he returns her affections. They arrange to be married, but Noah Percy (Adrien Brody), a young man with an apparent developmental and learning disability, stabs Lucius with a knife because he is in love with Ivy himself. Noah is locked in a room until a decision is made about his fate.
Edward goes against the wishes of the other Elders, agreeing to let Ivy pass through the forest and seek medicine for Lucius. Before she leaves, Edward explains that the creatures inhabiting the woods are actually members of their own community wearing costumes and have continued the legend of monsters in an effort to frighten and detract others from attempting to leave Covington. He also explains that the costumes are based upon tales of real creatures who once lived in the woods. Ivy and two young men (unaware of the Elders' farce) are sent into the forest, but both protectors abandon Ivy almost immediately, believing the creatures will kill them but spare her out of pity. While traveling through the forest, one of the creatures suddenly attacks Ivy. She tricks it into falling into a deep hole to its death. However, the creature is actually Noah wearing one of the costumes found in the room where he had been locked away after stabbing Lucius.
Ivy eventually finds her way to the far edge of the woods, where she encounters a high, ivy-covered wall. After she climbs over the wall, a park ranger named Kevin (Charlie Hofheimer) spots Ivy and is shocked to hear that she has come out of the woods. The woods are actually the Walker Wildlife Preserve, named for Ivy's family, and it is actually the modern era instead of the 19th century as the villagers believe. Ivy asks for help and gives Kevin a list of medicines that she must acquire, also giving him a golden pocket watch as payment. During this time, it is revealed that the village was actually founded in the late 1970s. Ivy's fatherâthen a professor of American history at the University of Pennsylvaniaâapproached other people he met at a grief counseling clinic following the murder of his father and asked them to join him in creating a place where they would sustain themselves and be protected from any aspect of the outside world. When they agreed, Covington was built in the middle of a wildlife preserve purchased with Edward's family fortune. The head park ranger, Jay (M. Night Shyamalan), tells Kevin that the Walker estate pays the government to keep the entire wildlife preserve a no-fly zone and also funds the ranger corps, who ensure no outside force disrupts the wildlife preserve.
Kevin secretly retrieves medicine from his ranger station, and Ivy returns to the village with the supplies, unaware of the truth of the situation. During her absence, the Elders secretly open their black boxes, each containing mementos from their lives in the outside world, including items related to their past traumas. The Elders gather around Lucius's bed when one of the townsfolk informs them that Ivy has returned, and that she killed one of the monsters. Edward points out to Noah's grieving mother that his death will allow them to continue deceiving the rest of the villagers that there are creatures in the woods, and all the Elders take a vote to continue living in the village.

Question: Who killed one of the monsters?
Answer: Ivy