Hilda: A Netflix Original Series Wiki
Cauldron Island on a map.
Cauldron Island as seen in "Chapter 7"

Cauldron Island is a large Kraken, mistaken for a small island located in the Björgfjord near the harbor of Trolberg. It appears in "Chapter 7: The Lost Clan", "Chapter 7: The Beast of Cauldron Island, and "Hilda and the Great Parade". The name of the island was first revealed in the novelization.

Cauldron Island is surrounded by tall rocks on all sides, with a valley in the middle. The island is home to one of the very few Lindworms still living near Trolberg. She has made the entire valley her own personal garden, with all sorts of flowers. Hilda, Frida and Alfur came to the island to ask the Lindworm to burn a contract that could end the banishment of the Bragga Family.

In Season 2, in Chapter 7: The Beast of Cauldron Island, it is revealed that Cauldron Island is actually a colossal Kraken, after Erik Ahlberg fires a single harpoon into its rocky back.