HistoryCaer Colur, the horn washed up on the shore near Taran and Eilonwy, who were exchanging pledges to remember each other during their coming House of Llyr. That dynasty had ruled from Caer Colur and had woven many wondrous enchantments there. She gave the horn to Taran to remember her by.
After Taran defeated Morda, the dwarf Doli of the Fair Folk noticed the horn and said it was Fair Folk craftsmanship. He inspected the item carefully and informed Taran there was but one "call" left in it -- a magical summons of the Fair Folk, for urgent aid.
Taran debated using the horn during his encounter with Dorath but deferred the chance, and so lost his sword and his dignity. Later, when Craddoc was stranded on a perilous ledge, Taran sounded the horn and so called the Fair Folk to rescue the crippled shepherd.