The horn featured in the original score for the Christopher Nolan film Inception, composed by Zack Hemsey. The horn takes the form of a mammoth trombone backed up by heavy percussion (often repeated multiple times for maximum results of 'Inceptionation') that is played at intervals throughout the course of the composed piece of music to create a more than dramatic atmosphere, giving the characters a valiant amount of purpose.
Side effects of frequent exposure to the Inception Horn may include "the dream collapsing", "checking your totem" and everything happening in slow motion.
Leonardo Dicaprio: "This was not part of the plan!"
Joseph Gordon-Levitt: "The dream's collapsing!"
Inception Horn: "BWAAAAOORRRRRRRR."
Member of Audience: "OHHHH BOYYYYYY"