The Horn of Cathbhadh

The Horn of Cathbar was a work of magic, created so that High Priestess individuals could call upon the spirits of the dead as part of a ritual. Thinking about his father, Arthur took Merlin to a circle of stone and called upon the spirit of his father using the horn. It opened a gateway into the spirit world.

In the same way, Gaius believed that the horn would close the gateway as well as open it. Using a spacial potion, a remedy of Gaius', Merlin and Arthur searched for Uther's spirit, intending to use the horn to return him to the land of the dead. Before Uther could reveal Merlin's secret, Arthur blew the horn and his father disappeared. (The Death Song of Uther Pendragon)