Duke Sigmund Igthorn is the main antagonist of Disney's 1985-91 television series Adventures of the Gummi Bears. Duke Igthorn was once a knight of Dunwyn who was exiled for conspiring against King Gregor. At an unknown point afterwards, he took control of a rundown castle called Drekmore and its resident pack of Ogres. Since then, he has constantly conspired to usurp King Gregor and rule Dunwyn himself. He has a twin brother, Victor Igthorn, who is a brave knight who fights evil. It is implied at several points that Duke Igthorn's father had attempted to control Dunwyn, and raised one of his sons to do the same.
Igthorn is very bad-tempered and a schemer. He is competent enough to gain temporary success in many of his plans, but his success is always short-lived. He later develops a crush on fellow villainess Lady Bane, but his feelings are unrequited. While conquering Dunwyn is his regular goal, he has entertained grander ambitions when they feel possible: upon seeing the vast technologies of the magic Gummi city of Gummadoon, Igthorn sees seizing it as his chance to conquer the world.
Despite the fact that he has many Disney Villians' attributes, such as being full of himself, Duke Igthorn is also somewhat cunning and a fairly skilled commander. Because of his superior physical strength and command of Ogres, the Gummi Bears, as well as King Gregor, consider Igthorn to be a serious threat to their respective homes. Part of that threat is that Igthorn, apart from using Gummi technology such as mass-producing Gummiberry Juice for his forces, has access to various magical items that he is apparently able to purchase from a catalogue among other sources.
Igthorn is one of only about a dozen humans, who know of the Gummi Bears' existence. While he generally prefers to keep it that way so he may obtain Gummi Bear secrets for himself, he has discussed the matter with others such as with the mercenary Flint Shrubwood, whom he commissions to capture a Gummi Bear, and Lady Bane, who desires Gummi magic for herself. However, in the series finale, "King Igthorn," he reveals that he is convinced that King Gregor has knowingly allied with the Gummi Bears, considering how many times they have protected his kingdom. The fact that Gregor thinks that Igthorn is mad for such an apparently prepostrous idea, is maddening itself to Igthorn.
Duke Igthorn serves as the series' primary antagonist. He leads a group of Ogres, who are extremely loyal and physically strong, but very stupid. However, Toadie, a smaller and slightly more intelligent Ogre, serves as his lieutenant and personal bootlicker.
In the series' premiere episode "A New Beginning", Duke Igthorn is using a gigantic catapult to tear down Castle Dunwyn. When the Gummi Bears attempt to thwart Igthorn's plan, they inadvertently expose themselves to him and thus prove their existence. From that point forward, Igthorn makes it a point to try and capture the bears in order to torture secrets out of them, as the ancient Gummi technology could be advantageous in getting Igthorn to capture Dunwyn.
Often the main purpose of Igthorn capturing Gummi Bears, is in order to gain a steady supply of Gummiberry Juice, as it provides humans and Ogres with temporary super strength. Initially, Grammi Gummi, the primary holder of the secret recipe, is able to trick him multiple times with sabotaged preparations, but Igthorn eventually grows wise to that and uses test subjects before accepting another forced preparation.
In the episode "King Igthorn" (which is considered to be the show's series finale), Duke Igthorn suddenly disappears without explanation, and Castle Drekmore lies unattended for nearly a year. Believing him to be gone, the Gummi Bears send a message to the Great Gummies across the ocean telling them that it is safe to return. However, Duke Igthorn suddenly reappears, destroys Gummi Glen, and conquers Dunwyn. Luckily, the Gummi Glen Gummies, with help from the Ursalia Barbics, are able to stop him. Meanwhile, the heroes free King Gregor and the Knights of Dunywyn to drive him back to Drekmore.
Undaunted, Igthorn plans to immediately retake Dunwyn. However, a squad of Glen Gummies sabotage his means to do so, his secret Gummiberry Juice facility inside in his castle, and send the whole castle literally rocketing into the sky to its likely destruction, while Igthorn's Ogres scatter. At that, Igthorn's threat is permanently neutralized, with him left with nothing but Toadie as his follower.
- Igthorn is one of a relatively small group of humans who find out about the existence of Gummi Bears during the course of the series. Other humans that become explicitly aware of the bears, are Cavin, Princess Calla, Lady Bane, Sir Gawain, Princess Marie, Angelo Davini, Flint Shrubwood, the Hunter, Rotocelli, Doctor Dexter, Marzipan, Lord Willoughby, Prince Yen Moon and his subjects, Carney's assistant from the episode "Never Give a Gummi an Even Break", and the boy who turned himself into a Giant, from the episode "When You Wish Upon a Stone".