A poppet is a doll created as a representation of a person with the intention of casting dark spells upon that person by way of inflicting those spells upon the doll. Magical poppets are very similar to Voodoo dolls. Poppets have been used by magic practicers on many different occasions.
Mary Collins' poppetEdit
Mary Collins used a poppet made of straw to murder Lady Helen of Mora so that she could take on her form to get inside Camelot. The witch chanted a spell three times ("Acwele seo mægþ!") while stabbing the effigy. After having put a glamour on herself, Mary Collins went to Camelot and took the poppet with her, for unknown reasons.
When Merlin went to her chambers to give the singer one of Gaius' potions, he saw the poppet and was probably attracted to it by its magic, for ghostly whispers could be heard sounding from it. The young warlock picked up the effigy and studied it for a while and then turned his attention to Mary Collins' grimoire. He had to quickly put it down when he heard the witch coming, and covered up the evidence that he was snooping. When she entered the room, he told her he was delivering a potion to her from Gaius, and almost noticed the reflection of her true form in the closeby mirror, but she prevented him from seeing it (The Dragon's Call).
Morgause, disguised as an old woman, gave Morgana a bracelet with a Phoenix Eye telling her to present it to Prince Arthur as a token of her good wishes for his journey through the Perilous Lands to retrieve the Fisher King's trident. She than had to make a likeness of Arthur and bind it to the gift (the bracelet) using the skills that Morgause had taught her.
In her chambers, Morgana created a doll with pieces of wood and cloths, which she then burnt on a candle. As soon as the poppet was reduced to ashes, the witch put it in a metal box and lit it again with her magic. Morgana didn't have to chant a spell to set the effigy on fire (something she probably wouldn't have been able to do, considering her inexperience) but only concentrate her energies on it, as Morgause had taught her. The spell caused the Eye on the Phoenix bracelet to glow and to start consuming Arthur's life force, draining him of his energy and causing him to struggle through his exhausting jouney across the Perilous Lands.
Morgana was forced to repeat it twice because she was interrupted by her maid Gwen, who smelt burning coming from inside the chamber and wanted to know what it was. Probably, the Eye could only work if the poppet's ashes, that Morgana kept in the metal box and relit with her magic, were burning (The Eye of the Phoenix).
Merlin, under the effects of an ageing spell and disguised as "Dragoon the Great" or "Old Emrys", enterd the castle of Camelot, which had been conquered by Morgana, and the Queen's chambers to cast a powerful spell on a poppet ("Ontende eallne þæs drycræftes hire sawle!"). This enchantment caused the magical doll to burst into flame and start emitting faint ghostly whispers at the edge of awareness. The fire scorched but did not consume the effigy. Emrys hanged it under the High Priestess' bed once the fire went out.
Merlin's curse on the poppet had the effect of blocking Morgana's powers, making her unable to use magic. In fact, when Arthur and his followers entered the throne room to face off against Morgana and her men, Morgana tried to cast a stunning spell to kill Arthur once and for all, but as she pronounced the incantation, her eyes didn't glow yellow and the poppet's whispers could be heard. While the Witch is trying to focus her energies to activate the spell, Merlin concentrated his powers on the poppet, strengthening the enchantment, and forcing Morgana to retreat due to her being defenseless and under shock that her magic had somehow suddenly stopped working. Morgana knew that Emrys was responsible for the prevention of her magic powers, as she had noticed him wandering through the castle earlier, but had failed to find him (The Sword in the Stone: Part Two).