Poultice

A poultice is a enchanted bag of herbs and other magical ingredients, often applied to the body to relieve soreness and inflammation.

History
Merlin created a magical poultice to cure Gwen's father when he was struck by the plague. Merlin's poultice was activated by the recitation of an incantation ("Þu fornimest adl fram guman!"), at which the poultice released a steam of medicinal vapours. When Arthur's soldiers searched Gwen's house looking for evidence of a possible use of magic, they found Merlin's poultice, which was magically glowing, under Tom's pillow. Because of this Gwen was arrested and accused of witchcraft (The Mark of Nimueh).

Morgana also sneaked into Arthur's chamber and planted a magical poultice under his pillow to frame Gwen for enchanting the Prince. Gaius said that this poultice bore the symbols of the Old Religion, and it was possible to cast spells with it. When Merlin decided to cast an ageing spell on himself in order to being caught and arrested and, by doing so, saving Gwen, he had Gaius make an identical poultice to the one used by Morgana. Disguised as an old sorcerer, Merlin entered Arthur's bedroom and, knowing that the Prince was listening, he chanted a fake spell while holding the poultice in his hand before being stopped and arrested (Queen of Hearts).

Trivia

 * Both times a poultice has appeared it results in Gwen being arrested.