The horn was Susan's second gift from Father Christmas in 1000 NT. It is enchanted, and if it is blown in a time of need, help will come. Susan blows the horn when she and Lucy are being attacked by the White Witch's wolves, and Peter hears the call and comes to their rescue.
During the chase of the White Stag that eventually leads to the Pevensies return to England, Susan accidentally loses the horn in the forest. It is later found by Dr. Cornelius, who gives it to Caspian when he has to flee for his life. Caspian later sounds the horn, summoning the four Pevensies back to Narnia.
Caspian later leaves the horn with Trumpkin in case of an emergency when Caspian sails to find the lost Lords. It is kept in Cair Paravel as a high treasure of Narnia, and it is not known if it is ever blown again.