When Mor'du was human, he had once possessed a close familial bond with his father and three younger brothers. However, he was confident in his strength but confused this for character, and thus led to his arrogance, a trait that caused him to struggle with his loved ones. Like Merida, he let his pride control his actions and clashed with his family, but unlike Merida, he was willing to let hatred and desire get the better of him instead of realizing his mistake and mend what was left of his broken family. His desire for power drove him to go far enough to start a violent war, and then ultimately killed his own brothers in cold blood.
As a black bear, Mor'du's hatred and rage manifested through his actions and carnage, especially when he slaughtered his remaining family. However, after fighting his own men, his violent nature would eventually consume what was left of his humanity, leaving his human mind and conscience buried deep within the mind of his cursed form. As a result, Mor'du was left as a vicious, malevolent monster incapable of sapience. His uncontrollable violence was so prominent that even a young woman could fall victim to his murderous intentions, and all of Scotland knew and feared him.
When he was finally killed, the spirit of the eldest prince was finally released and, apparently learning from his tyrannical mistakes and what his intense lust for power had turned him into, thanked Merida with respect for releasing him and moved on peacefully to the afterlife; becoming a will-o-wisp.
As a human, Mor'du was powerfully built and was the tallest of his brothers. He had brown hair and a beard and wore a dark colored robe and fur cloak. He did not wear a shirt, which showed that his body was covered with various tattoos. He also had unnaturally glowing yellow eyes.
As a bear, Mor'du is completely black, very large and is taller than King Fergus. He has many hideous scars and wounds across his body and the broken shafts of arrows, spears and axes from many past attempts to kill him sticking from his upper back, proof of all the murder he has committed over the years. This makes him appear malformed and rugged, but still very large. His face is the most disfigured of all and has one glowing yellow eye and one dead red eye and a crooked lower jaw. Thanks to the potion he drank, he is granted with the strength of ten men; he was able to stand against the greatest warriors of four Scottish clans and defeat them all with little effort, despite Scottish people being among the finest warriors in the world. The potion also granted him a degree of resilience to injury as he doesn't seem bothered by weapons stuck on his back, along with a strong hide. King Fergus' sword was said to have shattered when he swung it against the beast's side and when Merida attempted to fend him off with an arrow to the forehead, it splintered into bits when it struck him.
Mor'du first appears at the beginning when King Fergus and Queen Elinor, with their child Merida celebrating Merida's birthday. When Merida fires an arrow off into the woods and goes to find it, the wisps appear and try to lead her with them. From the undergrowth, it is shown that Mor'du is watching her. When Merida finds herself back at the camp, her mother begins taking her away, but Mor'du attacks and Fergus with his household guard rush 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, even striking the head off a spear when Fergus tries to stab him. As Fergus draws his sword and goads the bear to attack, Mor'du strikes, cutting to black. Later, it is revealed that the sword shattered when Fergus struck the bear and his leg was eaten, giving him a great hatred of the monstrous bear.
When Merida shows her upset at having to marry one of the three lords' sons as her duty to the family, Queen Elinor tells Merida the story of four princes (one of which was Mor'du) who were to have the kingdom split for each, Mor'du plunged the kingdom into chaos by wanting to rule over them and be the best of them, suggesting that he waged war against his brothers. When Merida goes to the witch's cottage, the Witch takes Merida's pendant as payment and says that a Prince came to her and asked for the strength of ten men. When Merida asks if this changed his fate, the witch confirms it and shows her what he used as payment, a signet ring with two 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 brothers, 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 and doesn't even slow him down. 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 then reveals himself and Fergus yells "Kill it!", but the great bear simply swats all opposition aside. When Merida herself becomes endangered by Mor'du as he prepares to eat her, Queen Elinor attacks, using her claws and teeth to defend her daughter, but Mor'du, proving much older and more dangerous, beats her down with his vast strength. Elinor, seeing the menhir they damaged earlier, smashes Mor'du against the menhir and damages it more, but is struck down. As Mor'du stalks towards Merida and her injured mother, the great stone falls, crushing him.
The bear's claws protrude from under the menhir, but his death finally releases the Prince's tormented spirit and soul, who nods thankfully to Merida before he takes on the form of a wisp and disappears.
In the short film, the origin of Mor'du is explained: Mor'du was once a human prince, the eldest of four brothers, all the sons of a wise king beloved by his people. One dark autumn, the king grew ill. On his death-bed, the king requested the kingdom be divided among his four sons, so together they could be the pillars that would hold it together and maintain peace. Alas, his firstborn son felt that, as the eldest and the technical heir, he alone had the right to rule the whole kingdom and a seed of selfishness grew in him like a poison. To his brothers, he claimed his right to rule and demanded their obedience, shattering the bonds of their brotherhood by breaking the family stone.
Words turned to war. Brother turned against brother in a war fought on four sides that changed the kingdom forever. While the eldest prince commanded a powerful army, the battle remained a bitter stalemate. Starving for victory, the prince cursed his fate and stalked the woods until he came to an ancient circle of stones. The Will-o-the-wisps appeared and led him to a dark loch where, not far from the shore, was the cottage of a witch. He demanded from her a spell that could change his fate. The strength of ten men he desired, and to persuade her he offered his signet ring. The bargain was struck though the Witch could see his wounded soul. She completed the spell in the form of a potion but offered him a choice: To fulfill his dark wish or heal the bonds of the family he had broken.
The prince baited his brothers with a false truce, but when they met in their father's throne room he once again declared himself the sole ruler and demanded their obedience. When his brothers defied him again, he drank the spell, which immediately took effect. It gave him strength tenfold, but, to his surprise, it also transformed him into a great black bear. Only by mending the bonds torn by pride could he break the spell. Instead, he accepted the form of the monster and slew his brothers in cold blood.
He returned to command his army, but they saw only the dreaded beast and so turned against him. He slaughtered a great many while the rest fled the kingdom in terror. With the armies of the brothers fractured, the kingdom fell into darkness and ruin, and the blight of the Great Black, Mor'du, fell across his domain.
Mor'du doesn't appear on the show as of yet. However, Merida mentions his legend to Belle when wanting that she prepare a magical potion allowing to transform into giant bear, similar to Mor'du, to save her brothers.
- 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 that never speaks. 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.
- Mor'du's real name (from when he was human) is unknown, as he is known by this name as a demon bear.
- The scar on his eye resembles that of Scar from The Lion King, which represents the evil and darkness inside his soul.
- Also like Scar, Mor'du killed members of his family to seize power over his kingdom and caused desolation over it.
- 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.
- Both Mor'du and Merida also damaged their respective family heirlooms as part of declaring their separation, the carved stone and the tapestry respectively, and later regretted what they had done. Mor'du's effigy on the carving appears to have been viciously clawed, implying that he hated what he had done.
- 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.
- It can be presumed that he followed her because, 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.
- In another point of view, the Will-o'-the-Wisps were leading Mor'du to his own fate - his own desired death, given his thankfulness to Merida.
- Weapons do not seem to hurt Mor'du at all, 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, and an arrow straight to the head didn't even slow him down.
- 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. Whether or not this is actually true is unknown.
- 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.