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 gray root, similar to a straw voodoo doll. Morgause, 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 æwþ 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.
Every night Morgana had to return to Morgause's cave and cover again the mandrake root with the potion. One night she was seen by a guard who thought she was bleeding, and she was forced to kill him.
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.
The root was in the end destroyed by Merlin, who threw it into some flames. Without the root, Morgause's enchantment no longer held and Uther recovered (The Tears of Uther Pendragon).
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.