An Erumpent horn is a powerful magical object that comes from the Erumpent. The horn contains a deadly fluid that causes whatever it is injected into to explode. They are highly prized as potion ingredients, but are listed as a Class-B Tradeable Material.
Luna Lovegood's home was destroyed by an Erumpent horn that her father Xenophilius claimed to be the horn of a Crumple-Horned Snorkack. He proudly displayed it despite the warnings given to him by Hermione Granger, and he accidentally blew it up while trying to stop her, Harry Potter, and Ron Weasley from escaping Death Eaters shortly after Christmas in 1997.
Behind the scenes
In the first film adaption of Harry Potter and the Deathly Hallows the horn is not seen exploding. However after Harry Potter, Hermione Granger and Ronald Weasley escape by Apparition a flashing light followed by an explosion can be seen from the outside.