After Merida learned that the old woman in the cottage that the Will O' the Wisps led her to is a witch she asked her to give her a spell; to change her mother so she doesn't have to marry one of the lords' sons. The witch knew what might happen if she gave her one and the side-effect it would have if she had a fight with the one she wants to use it on or herself and tried to chase her out; but Merida told her that she'll trade her necklace for all of her bear carvings if she gave her the spell.
The witch agreed and used her magic to change her carving room into a conjure room so she could make it. She told Merida that the last time she made such a spell was for a prince, so he could have the strength of ten men as he gave her his ring as payment. Merida wasn't sure why it took the form of a cake, but the witch told her that "it would do the trick". As Merida was leaving with it, the witch suddenly remembered something about the spell and as Merida turned to ask her about it, she and Angus were back at the Ring of Stones.
After Merida returned to the castle, she placed the cake on a plate and tray (with some berries, a thistle and tea) before bringing it to her mother; but Elinor walked into the kitchen and told Merida that she was worried sick about her. Merida was starting to have second thoughts until her mother told her that "a decision still has to be made" and handed her the cake, by claiming that it was a peace offering that she made herself: nothing happened after the queen took one bite of it and started to feel sick. Merida wasn't happy of the results as the spell's magic kicked in and turned her mother into a bear, so the two headed to the Witch's Cottage so the witch could undo it, but not before leaving Harris, Hubert, and Hamish in the kitchen with the half eaten tart. When the two got to the cottage, the witch was no longer there, but she did leave Merida a message about the spell in her cauldron: by the second sun rise Elinor will stay a bear forever if Merida doesn't "mend the bond, torn by pride."
Merida wasn't sure what it meant until the wisps led both her and the queen to the ruins of the "Ancient Kingdom" from the old legend and learned that the oldest prince was the same prince that the witch had told Merida about and that the spell had turned him into the demon bear, Mor'du. The ruins also had a split stone carving of the brothers: just like the Family Tapestry she 'split'. Merida knew what she must do: fix the tapestry and place it over her mother before the second sun rise and before she ends up like Mor'du, forever.
But after King Fergus saw Elinor's torn dress and the 'bear' in the castle, he locked his daughter in the tapestry room to keep her safe, while he and the lords went to kill 'it'. As Merida cried for Maudie to unlock the door, three bear cubs (walking on two legs) came up to the lock door and she knew right away that her brothers' sweet teeth got them to eat the rest of the 'spell'. But the three were still able to free their sister and help her save their mother. After Merida and Elinor defeated Mor'du and freed the prince's soul in the stone ring, the sun started to rise and Merida placed the fixed tapestry over her mother to change her back, but nothing happened and Merida started to cry. As Merida told her mother how much she loves her, that she was sorry for what she did and that she knew that she was always there for her, she stopped crying when she felt her mother's hand stroking her hair. Their forgiveness and love for each other undid the spell on both the queen and the triplets.
It may not look like it, but the spell did change Elinor and help Merida understand the true meaning of fate as it made the bond between them stronger than ever.
The story of how the oldest son of the Ancient Kingdom came across the spell that turned him into Mor'du is told by the one who gave it to him in The Legend of Mor'du.
When the strong prince felt the need to change his fate so he could have the throne all to himself, he found himself in a ring of stones and was led by the wisps to a cottage that is home to a witch. When he demanded for a spell that will give him the strength of ten men as he gave her his signet ring, the witch saw the torment in his soul and gave him the chance to fulfill his dark wish or mend the bond he broke between him and his brothers as she placed the green liquid form spell into a large horn.
The prince baited his brothers into the castle's throne room before claiming his kingdom in a false truce and he drank the spell when his brothers protested. He felt the strength he asked for before the spell turned him into a great black bear, the witch had also given the "spell bracer" that would turn him back when he "mended the bond torn by pride", but he accepted his new form and defeated his brothers by slaying them in cold-blood.
When the prince returned to command his army, they only saw the beast and defended themselves as the monster took over their former leader's mind. As the surviving men fled from what remained of the kingdom, the ruins became the beast's domain as the former prince remained trapped in his hollow and scared shell, by choosing power over family he turned his changed fate into a grim one.
As the witch ended the story she asked Wee Dingwall if he "would challenge fate", before shoving the same kind of spell (in a different cake form) into his face, but he turned his head away from the cake as he told her that he only wanted a drink of water before running out of the cottage in terror.
In Once Upon a Time, it is known as the Spell of Mor'du. It is present in the episode "The Bear and the Bow". After Merida's brothers are captured by the Clan Lords Macintosh, MacGuffin and Dingwall, she recruits Belle to help her save them. The pair enter the Witch's Cottage where Merida uncovers a potion that will help her defeat the Lords. While Merida practices her archery, Belle makes the potion in a liquid form, though is hesitant to give it to her, claiming that she doesn't need magic to change her fate. When Merida does confront the Clan Lords, Belle switches the potion with a vial of water, though Merida is able to eventually defeat them anyway without the use of the potion, and save her brothers.
After the Dark Curse is enacted, Emma captures Merida's heart and takes control of her. In order to make Gold a true hero she orders Merida to kill Belle, in hopes that he will be willing to defend her. She forces Merida to drink the Spell of Mor'du potion and Merida transforms into a bear. Merida chases Belle, but sees Gold and confronts him. He is able to throw some anti-transformation powder into her mouth which undoes the effect of the potion and restores Merida back to her original state.
- In the video game Mor'du's blight twisted the witch's magic and made the spell turn Elinor into a bear: and instead of forgiveness and love, Merida had to clear the ancient Waystones to change her (and her brothers) back.
- The cake is seen in Merida's flashbacks of the events that led to her journey. Her getting it from the witch, the princes eating it and Merida handing the tart over to her Mother.
- In The Bear King, the witch possessed a concoction that could turn people into bears, whether or not this is the same spell is unknown.
- The cake resembles an eyeball, probably due to the strange ingredients of which it was made.
- Merida added blueberries as topping to the cake when she prepared it for her mom. Coincidentally, the latter mistaken nightshade berries with blueberries during her time as a bear.
- One of the ingredient for the cake is a newt. It is a reference to the cancelled Disney/Pixar movie Newt.