A legendary monster, Mor'du is an over 13-foot-tall bear with huge claws, a misaligned jaw, and long, sharp teeth. His body is covered in scars, broken weapon hilts, and arrows. One of the Production Designers of the film, Steve Pilcher, calls Mor'du a "Demon bear" and "Moby Dick on land".
However, Mor'du was not always a bear; he was originally a human prince, who wanted to rule the kingdom he shared with his three brothers alone, and went to a witch to gain "the strength of ten men" in order to take over the land, paying with the ring of his house, bearing the crest of his family, two crossed axes. The spell he received eventually transformed him into a bear, and he murdered his family, causing the fall of the kingdom. Mor'du lived on for centuries, and the bear side of him overtook his humanity, making him a monstrous beast.
Mor'du first appears in the film's prologue, when Merida, daughter of King Fergus and Queen Elinor, fires an arrow off into the woods and goes to get it back. Many wisps appear and try to lead her with them. From the undergrowth, it's suggested that Mor'du is watching her. After she returns to her parents, Mor'du attacks and Fergus with his household guard rushes to defend his wife and child. Though they give the princess and the queen time to escape on horseback, Mor'du shows no fear nor vulnerability to them. Later, it is revealed that Fergus had his leg eaten, thus beginning his fierce rivalry with the demon bear.
When Merida runs away from home after a fight with her mother, and follows the wisps to the witch's hut, the witch takes the medallion of clan DunBroch as payment for a spell to "change her mother". The Witch says that a prince once came to her and demanded the strength of ten men. When Merida asks if got what he wanted, the witch confirms it, showing Merida his clans ring with the crossed axes.
Later, Merida and her mother (now turned into a bear) follow a trail of wisps to a foggy ruin, bearing the crossed axes of Mor'du's family. Merida explores, falling after walking on an unsteady piece of rubble. Inside, she finds a throne room very similar to her own family's and a shattered stone carving of four brother, one of the brothers broken from the others. She slowly realizes that the Prince who asked for the strength of ten men indeed had his fate changed, and that man became Mor'du. Mor'du appears, stalking Merida from the shadows and he charges. Merida fires an arrow straight at his head, but the arrow does no damage whatsoever. Merida hurriedly tries to crawl up through the ruins and reach her mother's paw, with the gigantic bear trying to devour her. At the last second, she leaps and just manages to grab her mother's paw in time as Mor'du snarls and snaps after her. Merida and her mother run as fast as they can away, running to the great standing stones and bashing into them a bit harder, making a hairline crack along one of the great menhirs.
Merida realizes she must mend the bond torn by pride (as the Witch said), fixing a tapestry her mother had made of Merida and the family, thereby breaking the spell. However, her mother is attacked by King Fergus and chased to the ring of stones. As Merida rushes to save her mother and change her back, Mor'du stands from the shadows, following her. Mor'du attacks and her father and the lords fight against the bear, but fail. When Merida herself becomes endangered by Mor'du, Elinor attacks, using her claws and teeth to defend her daughter, but Mor'du, proving himself to be bigger and more dangerous, beats her down with his vast strength. Elinor, seeing the stone they damaged earlier, smashes Mor'du against the stone and damages it more, but is struck down. As Mor'du stalks towards Merida and her injured mother, the stone falls, crushing him.
The bear's claws protrude from under the stone, but his death frees the Prince's spirit, who nods thankfully to Merida before he takes the form of a wisp and disappears.
The prince sought to be given the strength of ten men as human. His transformation into a bear gave him the additional strength of a bear, but as the demon bear Mor'du his strength and aggression exceeded those of a normal bear. No human, even an entire clan, could stand against him; it appears only another bear could face him to some extent, but even then it would not be as strong. Despite his large size, Mor'du is extremely fast. He is also very enduring, having survived many attacks, and is still as ferocious despite his heavily battered body. As a bear, he has improved nightvision and a great sense of smell to keep tracking Merida.
- Mor'du's name may come from "mor" and "dubh", the respective Gaelic words for "giant" and "black", which appropriately describes his physical appearance and fur color. It may also come from "mortus", the Latin word for "death." "Mordu" is also French for "bitten," which is somewhat also suitable to Mor'du's character, for biting is the method in which he had removed King Fergus' leg.
- Curiously enough, this is actually reversed for the word "bear", which actually originates from the Old English word for "brown", which appropriately describes the animal's appearance.
- Mor'du is the first Pixar villain to never speak. His wish for the strength of ten men is the only known dialogue from him, but it is not heard from him directly.
- Mor'du was said to have bought a mahogany cheeseboard from the witch, along with his spell.
- After being Mor'du for so long, a real bear, the prince seemed to have lost his anger upon his release upon his death, evidenced by his thankful nod to Merida and Elinor for releasing him from his monstrous fate.
- In one point of view, the Will-o'-the-Wisps lead Merida to her mother to change her fate, but they can also be seen as leading Mor'du to his own fate, his own desired death (given his thankfulness to Merida upon his death).
- Mor'du's real name is unknown (from when he was human), as he is known by this name as a demon bear.
- Mor'du is completely different from who Merida is and what she did. Both clash with their families because of pride, but go in two different directions. While Merida wanted to be free and frequently clashes with her family, she cares for them and showed no hesitation to try and break the spell when she accidentally cursed them. Mor'du, on the other hand, only wanted to rule the kingdom because of himself being heir to the throne, and instead of realizing his mistake and breaking the spell he inflicted on himself, he allowed his pride to get the better of him and murdered his family without remorse.
- No one really knows why Mor'du wanted to kill Merida (or her family and possibly the Lords and their sons).
- However, it can be presumed that as the Will-o'-the-Wisps can lead anyone to their fate, Mor'du (having encountered the wisps before as a human) might have wanted to kill Merida as she is the one following the wisps.
- It can also be presumed that he wanted someone to kill him, as it seems that he's thanking Merida and her mother for felling the monolith before he turns into a Willo-the-Wisp. Possibly because he has lived for so long and wanting peace.
- Weapons do not seem to hurt Mor'du, the beast's hide has several spears and arrows sticking out of him, as his body is riddled with many scars and along with a scarred face all show he has been in many battles and committed many murders over the years. Also, Fergus' sword shattered when he struck the great black bear, which many weapons might have.
- Mor'du has fought with all four clans.
- Eurasian bears are brown (light brown to dark brown) but Mor'du is all black, possibly to represent the darkness in him.
- It is said in the song of "Mor'du" that he has devoured dozens, including babies. It is unknown whether this is actually true or myth.
- Mor'du has similarities to the vile Cambodian ruler Pol Pot. Just as Pol Pot did, Mor'du refused to take responsibility for the traditions and the lives of his people. On top of that, he allowed his nation to fall into destruction and decay, leaving behind a tragic legacy of misery and mass graves.
- It should be noted that Mor'du seemingly had a strange obsession with killing Merida specifically, as in every scene he was in, she was always the main target.
- As Elinor tells Merida the story of the ancient kingdom, the oldest prince can be seen roaring like a bear, foreshadowing the revelation that he is Mor'du.