In the days of the Old Religion, the High Priestesses would gather at the Great Stones of Nemeton each year at Beltane and use the Horn to summon the spirits of their ancestors. They trained for years before entering the Spirit World, and one thing they were told never to do was to look back at the spirit as the veil closed, as doing so would release it into the living world.
When not in use, the Horn was kept in the temple on the Isle of the Blessed. It was smuggled to safety when Uther attacked the island during the Great Purge and at some point entered Valdis's possession. Valdis later entrusted it to Arthur after he saved her from execution.
On the third anniversary of his father's death, Arthur traveled to Nemeton and used the Horn to summon his spirit. He entered the Spirit World and spoke with Uther, but as he was leaving he looked back at his father's spirit, releasing it into the living world.
Arthur later used the Horn to return Uther to the Spirit World. To do so, he had to blow the Horn in Uther's presence, as only the person who had summoned the spirit could send it back (The Death Song of Uther Pendragon).
- The Horn contains very powerful magic (The Death Song of Uther Pendragon).
- How the Horn entered Valdis's possession is unknown (The Death Song of Uther Pendragon).
- Before the Great Purge, Gaius used to take part in the High Priestess' ceremonies at Beltane (The Death Song of Uther Pendragon).