Whistle

During a high school basketball game, star athlete Mason "Horse" Raymore is haunted by visions of a burned man before mysteriously bursting into flames and dying. Months later, transfer student Chrys Willet inherits Mason's old locker and finds a skull-shaped whistle. When provoked by Mason's former teammate Dean, both he and Chrys are sent to detention by Mr. Craven, along with her cousin Rel, Dean's girlfriend Grace, and Ellie, a smart girl with diabetes.

After detention, Craven confiscates the whistle and blows it out of curiosity, while Rel later sneaks back to steal it. Unbeknownst to Rel, Craven is soon killed by a grotesque apparition of himself, who forcefully gives him massive lung cancer. Later, the detained students also blow the whistle, and its sound causes them to be haunted by their own apparitions.

The next morning, Craven's death is reported as extreme stage lung cancer, which baffles those who knew him since he exhibited none of the symptoms from the day prior. Chrys and Ellie later learn from Mason's occult-collecting grandmother, Ivy, that the whistle is an Aztec death whistle inscribed with the phrase "summon your death". Anyone who hears it becomes stalked and is eventually killed by a manifestation of their destined demise. Despite their attempts to warn the others, Grace dies after rapidly aging into an old woman, and Dean is later killed in a manner resembling a car crash. When Chrys, Ellie, and Rel visit Ivy on her deathbed, she reveals that the curse can be avoided by marking another person with their blood, while their respective demise is nearby, and transferring it to them before dying, Chrys also sees her supposed apparition, which represents overdosing.

Rel abducts violent youth pastor and drug dealer Noah as the church, and attempts to mark him with his blood, but Chrys and Ellie intervene, refusing to sacrifice someone else. Rel relents, but is soon killed by his apparition, which tears him apart like a junkyard shredder. Chrys and Ellie discover the curse can also be escaped by briefly dying and being revived, and Ellie has the medical supplies and equipment at hand having kept such in her car for emergencies. While attempting to stop their hearts and resuscitate each other, Noah escapes and confronts them, but after touching Ellie's blood, he inadvertently transfers the curse to him and he is killed by his own apparition.

Three months later, Chrys and Ellie are now together and believe the curse has ended, until new student Asha Nelson opens her locker and discovers the whistle inside. In a mid-credits sequence, Asha is called to the stage at a school assembly in the auditorium to play the school anthem on her violin to kick off the spring semester, but instead pulls out the whistle. Chrys and Ellie try to scream for her to stop, but she blows the whistle anyway, and dooms everyone in the auditorium.