Spirit World

The Spirit world, also known as the Underworld (or the Other World), is a region that may be under the surface of the earth or in a hypothetical parallel universe. It could be the place where the souls of the recently departed go, and this is possibly the place where heaven and hell are located. Anyway, it's a place of darkness and of ancient and evil spirits. The main creatures that dwell in the Spirit world are, in fact, the Dorocha, the numberless voices of the dead.

However, the Spirit world seems to be not only populated by evil creatures, for the Vilia, benevolent spirits of the brooks and streams, come from the Underworld.

The Spirit world is separated from the world of the living by the Veil. The Cailleach is the gatekeeper to the Spirit world and of the Veil (The Darkest Hour).

The Pool of Nemhain is the last of the five Gateways that separate the world of the living from the world of the dead (Lancelot du Lac).

Some spirits that were badly wronged cannot find peace in the Spirit world. They try to find their proper place in the Underworld, but the unjust nature of their death has denied them that. Because of this, the druids built shrines to bind them to the earth (A Herald of the New Age).

The manticore is a creature that lives in the Spirit world, and cannot prolong its existence in the world of the living. Its live source is in fact the ancient evil that exists in the Spirit world. To come into this world, the Manticore has to use a portal (Love in the Time of Dragons).

Another creature that dwels in the depths of the Underworld is the Fomorroh, who can be conjured into this world but has to be kept in the dark, for its not used to be out in the light (A Servant of Two Masters).

Trivia
In a deleted scene from "The Secret Sharer" Morgana, having been badly injured by Merlin's whirlwind spell in the previous episode and lying on the brink of death, is about to enter the Spirit world. There she sees her sister Morgause, still with injuries on the right side of her face and surrounded by the spirits of the dead (people dressed in grey and with their hands extended to her). Morgause gives Morgana a magical coin, "the price of a soul" (as the Witch describes it to the Dochraid in "Lancelot du Lac") that is able to revive the High Priestess and let her leave the Spirit world, healed. Morgana will use the coin, throwing it into the dark waters of the Pool of Nemhain, to summon Lancelot's shade (Lancelot du Lac).