The mandrake is a magical plant used frequently in spells and rituals. The root was also thought to scream when it was dug up from the ground, killing everyone who heard its cries. In the show, the mandrake is a big and grey root, similar to a straw voodoo doll.
Use on UtherMorgause, with the help of her half-sister Morgana, used it to enchant Uther Pendragon and make him lose his mind. Morgause prepared a potion in a big cauldron, a thick, dense, black liquid similar to dark mud, and threw a handkerchief, which had the tears of Uther on it with the root itself in it that sank in the potion. While doing this, the mandrake emitted a piercing scream that only those with magic could hear. For those without magic, Morgause explained, the root would pierce the very recesses of the soul, twisting the unconscious into the very image of fear and dread. She then pronounced a spell ("Mid þæm wundorcræft þæs ealdan æwe ic þe hate nime Utheres wopdropan ond þa gemengan mid his blod. Sy he under wittig ond deofol seocnes his heorte afylþ") and took the root from the cauldron. Morgana placed it under Uther's bed, and the liquid started to drip from the mandrake.
The spell caused Uther to hallucinate, seeing people who had died because of his choices and actions before and during the Great Purge. These included his wife Ygraine frightened and bleeding, begging Uther not to harm her and an unknown drowned boy, who was revealed by Gaius to be one of the many children Uther had killed simply because of the magic they had inherited from their parents. The spell gave the impression to the people of Camelot that their King was losing his mind.
Morgana also worsened Uther's sickness by wrapping the mandrake up with a string and pulling it twice: this caused the root to scream, like if it was in pain, and so Uther had another vision of Ygraine with many drowned boys, begging the King not to harm them.
Use on Gwen
In a wicked plan to brainwash Queen Guinevere and turn her against Camelot, Morgana kidnapped her former maid and took her to the Dark Tower, a place of terror that haunted the hearts and minds of men. She held the Queen locked in a dark, gloomy room filled with a large number of mandrake roots hanging from the ceiling, each of them covered with the black, muddy potion that dripped off them.
The cruel magic of the mandrake roots pierced the depths of Gwen's soul, twisting her unconscious into the very image of fear and dread. The woman spent an undefined period of time (probably a few days) being haunted by visions of Elyan, Merlin, Gaius and Arthur, taunting her and teasing her, while hearing ghostly screams of terror and other spirits laugh at her.
Morgana paid Guinevere a few visits, one time even to get her to dine with her, to show her how nice and friendly she was compared to the evil people in her visions. The High Priestess, with her gentile manners and desire to lovingly take care of the Queen, induced Gwen to believe that she was all she had left in the world, her only friend, the only one she could trust, while her other former friends hated her and taunted her.
So strong was the magic of the mandrake roots, along with Morgana's deception, that, although Gwen turned against the sorceress a couple of times, in the end she was completely taken in by her mind game. Even after she was rescued by Arthur and taken back to Camelot, Guinevere was still under Morgana's influence and she had become her ally, sharing the same hatred towards everyone in Camelot (The Dark Tower).
It was later revealed, in the episode "With All My Heart", that Morgana had performed on Gwen an ancient ritual of the Old Religion called the "Teine Diaga", the "sacred fire". According to Gaius, the ritual used the mandrake root to bring unimaginable terror to the victim (their screams could be heard twenty leagues away). When it was finally over, their will was no longer their own, they were slaves of the High Priestesses for eternity. As a boy, Gaius was privy only to rumors, for such mysteries were revealed only to a handful of female initiates, destined to become High Priestesses. At Merlin's time, the only people who truly knew the Old Ways and the Teina Diaga were Morgana Pendragon and the ancient Dochraid.
Merlin, disguised as Old Emrys, went to the old hag's cave to ask her how to break the sorceress' spell. The Dochraid refused, since Emrys was no friend of the Old Religion or of Morgana. Merlin threatened her with Excalibur, the only weapon which could kill a creature of the earth such as the Dochraid. With the sword at her throat, the hag told Merlin that Guinevere was doomed, her spirit consumed by the Teine Diaga and bound to the Silver Wheel for all eternity. Her body was nothing but an empty vessel, filled by Morgana's will.
To free the Queen's spirit, a thing that only the most powerful sorcerers could hope to attempt, Merlin would have had to travel to the Cauldron of Arianrhod and there use all his powers to summon the White Goddess herself. The waters of the Cauldron held the Goddess' powers and only their touch could heal her, but Gwen would have had to enter the lake willingly; if she were tricked, forced or beguiled, she would fall into the abyss and be lost forever.
- Although never confirmed or explained, it is possible that when a mandrake's effects on a person are worsened, it can partially reduce a person's consciousness and focus the victim more on their visions than their surroundings. In fact, when Uther's sickness was worsened, he was so lost in visions that he was unaware that Morgana was standing in front of him even when he was awake.
- According to Katie McGrath (Morgana), in the audio commentary of Series 3 Episode 1 (The Tears of Uther Pendragon: Part One), she and Emilia Fox (Morgause) were so fond of the mandrake root they nicknamed it "Manny".
- The enchantment cast by Morgana on the many mandrake roots used to traumatize Gwen must have been different from the magic worked by Morgause, because the Witch hadn't probably used the Queen's tears and she didn't have to continuously recover the roots with the black potion.