Uther Pendragon in the Spirit World.
Spirits are apparitions of the deceased that can appear to, interact with, and possess the living. Most of the spirits that appear in the series are human, but places and elements can also have spirits associated with them.
Becoming a Spirit
Freya fulfills her promise to Merlin.
Humans (both magical and non-magical) appear to become spirits automatically after death. Most spirits seem to reside exclusively in the Spirit World, but they are capable of existing outside of it (The Coming of Arthur, A Herald of the New Age, The Death Song of Uther Pendragon).
The spirits of some magical people are unable or unwilling to enter the Spirit World after death, in most cases because they have unfinished business among the living. Freya, for example, became the spirit of the Lake of Avalon in order to fulfill her dying promise to Merlin (The Coming of Arthur). Similarly, the Shrine Boy's spirit was unable to find peace until after he had avenged his death (A Herald of the New Age).
The spirits of non-magical people are also able to exist outside the Spirit World, though they can only exit it with the help of spells, necromancy, or magical objects such as the Horn of Cathbhadh (The Sins of the Father, Lancelot du Lac, The Death Song of Uther Pendragon).