The Ursalia Horn is a giant horn located in the ancient Gummi capital of Ursalia, that can cause major destruction with a single blow. Ursa, in her desire for revenge against the humans who drove the Barbics from their home, seeks out this weapon, intent on wiping out the entire human race, though not knowing what the weapon looks like.
It is accidentally discovered by Buddy and Sunni, who use it to warn the other Gummies of an attack by Duke Igthorn and his Ogres. The weapon causes damage throughout Ursalia, causing Igthorn to temporarily flee.
Despite Zummi's insistence that the weapon is too powerful to control, and should never be used, the Barbics seek to use it against humans by any means necessary. This leads to a drawn-out battle between the two Gummi clans over control of the weapon, before Igthorn captures Buddy and Sunni and demands the weapon as ransom for their return.
The Gummies bring the weapon out of the city to Igthorn, who predictably reneges on his promise to return the cubs. The Gummies then launch a surprise attack from within the horn, retrieve the cubs, and defeat Igthorn. Finally realizing that the weapon is too powerful if it ever fell into the wrong hands, Ursa orders it dismantled, never to be used again.