On (怨, Malice) is one of the defining features of the movie Hunter × Hunter: The Last Mission. It is the opposite of Nen. Instead of utilizing life energy, the aura from On is derived from hatred and can be considered "death energy." It grants one immense strength, with each user being able to utilize abilities similar to Nen at 100% efficiency. Unlike Nen, this aura is said to be stronger than Nen, and has no categories; meaning, one who masters On has the ability of an Enhancer, Transmuter, Conjurer, Emitter, Manipulator and Specialist. In the movie, On can be used after having contact with Jed's blood and making a contract to obtain On. However, the use of On will kill the user after the user breaks its contract (or, most likely, fails the contract). Jed, Gaki, Shura, Rengoku and Gon are all users of On in the movie.
- It is interesting to note that the kanji used for "On" is the same superimposed onto Gon and Killua the first time they felt Neferpitou's aura. This has led some fans to speculate that Neferpitou is a user of "On."
- A dark aura surrounded Netero after he was defeated by Meruem, when he spoke about humanity's power for evolution, right before activating the Poor Man's Rose, and the King felt intense fear at that moment. It is possible Netero had used On at that point, although the aura could have been just a graphic effect.