The Horn of Hypnos is a mystical yellow/orange bulky mystical horn that turns its victims into mindless zombie-like "Trance Marchers", and it is one of the many artifacts featured in the W.I.T.C.H. series including the W.I.T.C.H. animated TV series.
The Horn of Hypnos is a magic yellow/orange bulky mystical horn, which turns its victims into zombie-like "Trance Marchers". The Horn has a low pitch, which changes them. However, once a victim becomes a Trance Marcher, they lack the brain knowledge and speed of how they attack, they can only follow orders the best they can and lack free will in general.
The Horn only works when it is pointed at its victim, hearing the Horn is not enough to be turned into a Trance Marcher. In order to prevent the Horn from working, two methods of escaping the spell are either to put in earplugs (Irma and Blunk did this), or to be underwater, since this seems to render the Horn useless (Caleb went underwater accidentally when the Horn was blown). When the spell is broken, it will vanish and reappear in a different location.
The Horn's debut was in Season 1: "Walk This Way", where Lord Cedric under orders from his master, used it to turn Drake and a few rebels from Metamoor into Trance Marchers. But Blunk stole it and everybody in Heatherfield fell victim to the Horn's spell, including Hay Lin. Caleb managed to break the spell and everyone turned back to normal and the Horn vanished.
It then reappeared in Season 2: "G is for Garbage" and the Knights of Vengeance used it to turn the W.I.T.C.H. (Guardians) into Trance Marchers, all except Irma, who escaped the spell. She enlisted Blunk's help and Blunk finally managed to sing the melody, that was required to break the spell and make the Horn vanish again to somewhere else. However, before the spell was broken, Trance Marcher Cornelia sent boulders flying in the air, one of them falling and smashing the Horn, ending its mind-controlling power.