The Discordant Horn is some sort of artifact that is a component of the evil demigod Vortek.
The horn was apparently a closely guarded relic of Pallam, but was lost when the coup of the royal family was conducted by the House of Azdragast. The Rain Knight stole the artifact from his family and delivered it to the Nightseeker as a show of good faith.
The horn was used to reconstitute Vortek, becoming a part of his mortal form.