In How to Train Your Dragon: The Hidden World, the safe place where dragons can live is called the Hidden World.
The Hidden World is a vast subterranean realm beneath the ocean. It is filled with glowing landscapes and provides a natural refuge where dragons can live beyond the reach of human hunters. Hiccup learns about it while searching for a permanent solution to the danger facing Berk’s dragons.
The location changes the story’s central question. Earlier films focus on whether humans and dragons can coexist in Berk, but the third film asks whether dragons may be safer living separately from humans. Hiccup eventually understands that protecting Toothless and the other dragons may mean accepting their departure.
The Hidden World is not the same as Valka’s ice-cave sanctuary in the second film. Valka’s refuge is a protected home for dragons, while the Hidden World is an entire hidden ecosystem and the ancient homeland sought by the dragons.