The Crown of Bogg (1982)

It is time to crown a new king of the kingdom of Bogg, and King Mildew's son, Milo, seems like the best creature for the job. However, Vandred, the king's brother, has other plans, and intends to put his son, Vandal, on the thrown. He challenges Milo, who must prove his worthiness by retrieving the Crown of Bogg and returning it to the kingdom. The crown has been banned from the kingdom for years because it is cursed, but Mildew and Milo are willing to face the risks of the quest in order to keep Vandred and Vandal from assuming control of the kingdom. But in order to retrieve the crown, they will have to leave Bogg and go to The Surface...where the humans are.