Ghost of the Black Lake is an episode of David the Gnome. It first aired 22 April 1985, and first aired in the US 20 June 1987.
David explains how gnomes make glass. He is called to the palace, where the king tells him about a little wizard named Tiraland who stole all of the gold in the castle. Now the trolls have thrown it into the eerie Black Lake. David has to ask the ghost of Black Lake Castle for permission to go to Black Lake. The ghost is a shapeshifter, and agrees to let David go, but for only three days. David locates the gold with the help of an otter. The trolls have tied a rope around the gold bag and are about to pull it up. The otter removes the rope from the gold and ties it to a big rock instead. The trolls chase David and Swift, thinking they have the gold. Finally, the trolls are scared by the ghost and give up the chase.