Duke Sigmund 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 brother, Victor Igthorn, who is a brave knight that fights evil. It is implied at several points that Duke Igthorn's father had attempted to control Dunwyn and raised his son to do the same.
In terms of personality, he 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.
Despite the fact that he has many Disney villian 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.
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, Toadwart, a smaller and slightly more intelligent ogre, serves as his lieutenant.
In the series premiere, Duke Igthorn was using a gigantic catapult to tear down Castle Dunwyn. When the Gummi Bears attempted to thwart Igthorn's plan, they inadvertently exposed themselves to him and thus proved their existence. From that point forward, Igthorn made 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 gives humans temporary super strength.
In the episode "King Igthorn" (which is considered to be the show's series finale), Duke Igthorn disappears from his castle for nearly a year. Believing him to be gone, the Gummi Bears send a message to the Great Gummis telling them that it is safe to return. However, Duke Igthorn suddenly arrives, destroys Gummi Glen, and captures Dunwyn. However, the Gummis are able to stop him.