Pentacle of Necromancy

A pentacle is a talisman or a magical object, typically drawn on the ground and inscribed with a pentagram or another figure, used in magical rituals.

Merlin, following the instructions of a grimoire called "The Art of Necromancy", drew one on the floor of Gaius' chambers, making it with silver paint. The circle had a spiral shape, with eight dots on the last line. The young warlock enchanted it with the spell: "Onluc þa soþan treow", and the circle glowed as if it was heated.

This pentacle allowed him to reveal the true essence a Shade that, thanks to Morgana's necromancy, had taken on the form of Sir Lancelot. When the knight walked into the pentacle, it emitted numerous ghostly screams and Lancelot's face was revealed as the one of a skeleton.

Although it wasn't used for an evil purpose, this form of magic can be considered as dark magic, for it's very close to necromancy, the most dangerous of all sorceries (Lancelot Du Lac).

Trivia
The pentacle Merlin created had almost the same form and drawing of the magical coin Morgause had given to Morgana, and that the High Priestess had used to raise the Shade. This is probably because the two sorcerers (Merlin and Morgana) had practiced the same form of magic, related to necromancy and Shade. It's unknown if this pentacle could be used to summon a spirit, too.