The Horn of Cathbhadh was a magical tool which had the power to summon the spirits of the dead. It was kept by the High Priestesses on the Isle of the Blessed, but when Uther attacked them, the Horn was smuggled to safety before the temple fell.
According to Gaius, who had taken parts in such ceremonies, long before the time of the Great Purge, each year at Beltane the High Priestesses would gather at the Great Stones of Nemeton and summon the spirits of their ancestors. In the days of the Old Religion, however, the priestesses trained for years before entering the Spirit World, since it was fraught with dangers. The one thing they were schooled never to do was to look back at the spirit, as the Veil closed. If they had done it, they would have released the spirit into this world.
Because of the pain caused by the untimely death of his father, Arthur decided to set off for the Great Stones of Nemeton and use the Horn of Cathbhadh to open the doors of the Spirit World, creating a gateway that allowed him to talk to Uther Pendragon. However, before returning to the world of the living, he looked back at his father, thus freeing his ghost into this world.
To force the dead king's spirit to return to the Other world, Arthur had to use the Horn of Cathbhadh to reopen the Veil between the worlds and blow the magical tool while at the ghost's presence. Only the person who had summoned the spirit could force it from this world (The Death Song of Uther Pendragon).