Coin of Necromancy

This coin was an artifact of the Old Religion and an extremely powerful tool of dark magic. It was given to Morgana by her sister Morgause. Morgana went to the Dochraid’s lair to seek advice on the use the magical coin. The Dochraid was one of the last creatures who remembered the time of the Old Religion and therefore knew its secrets. Morgana gave her the coin to inspect, telling her it was "the price of a soul", and the Dochraid was so enthralled by its power that desired to keep it, though Morgana swiftly refused. The Dochraid informed her that she would have to travel to the the Pool of Nemhain, the last of the five Gateways that separated the world of the living from that of the dead. Morgana had to cast the coin into its waters and focus her powers on it to raise a shade from the dead. Morgana did so, and the coin was lost in the depths of the Pool of Nemhain (Lancelot du Lac).

Trivia
In a deleted scene from The Secret Sharer, it was revealed that Morgause gave the coin to Morgana when the High Priestess had a vision of her in the Spirit world. She was lying on the brink of death following her encounter with the old Emrys, but after Morgause gave Morgana the coin, she was able to make the journey back to the world of the living, revived.

This magical coin had almost the same form and drawing of the pentacle Merlin used to reveal the shade's true nature. This is probably because the two sorcerers (Merlin and Morgana) had practiced the same form of magic, related to necromancy and shades.

The coin may symbolise the price that, like in Greek mythology, the souls of the newly deceased had to pay to be ferried by Charon to the other shore of the river Styx. In this case, both Morgana (in the deleted scene) and Lancelot had to pay for their journey back to the world of the living.