The legendary Horn of King Doric is a giant horn (see picture) built just north of the city of Rin and named after the great King Doric. It was built more than 200 years prior to the present day in the game (before the Great Northern Wall was built) as a last line of defense to protect the city of Rin.
Warning: The following text contains spoilers relating to the plot of Prophecies.
The Horn of Doric requires the mouthpiece (a regular size horn) known as Stormcaller to unleash its powers. It is unclear what powers the Horn of Doric possesses exactly, but in the Nolani Academy mission, when the Charr attempt to invade Rin and set it ablaze, Prince Rurik invokes the Horn which in turn stirs a rain storm that puts out the fires of the Charr,making them weaker (knowing that the fires make them stronger, because of their gods).