The Island of Tomorrow is a large island in the West of the Barbaric Archipelago in the book series. One hundred years before the birth of Hiccup Horrendous Haddock III, Tomorrow had been the location of the capital city and castle of the Kingdom of the Wilderwest. Grimbeard the Ghastly was the last King of the Wilderwest before Hiccup was crowned the new King in How to Fight a Dragon's Fury. When Grimbeard mistakenly killed his son, Hiccup Horrendous Haddock II, the island was abandoned and cursed.
The shores of the island are guarded by immortal, but blindfolded, druid warriors and the Dragon Guardians of Tomorrow (dragons of the upper atmosphere, cursed to be trapped in the beach sands of Tomorrow). No-one may land on Tomorrow unless they have collected the ten King's Lost Things. The Druid Guardian tests anyone who attempts to land on the island, and if they do not have the things, he summons the Dragon Guardians who carry the unfortunate victim up their death in the upper atmosphere. The Dragon Guardians are said to be visible today as meteors.
Hiccup learned about the history of the Kingdom of the Wilderwest during the time of Grimbeard the Ghastly and Hiccup Horrendous Haddock II from Excellinor the Witch while trapped inside her tree cell on Berserk. Hiccup learned about how the Isle of Berk became too small for Grimbeard and so he settled the Island of Tomorrow. Hiccup also learned that at the end of the Kingdom of the Wilderwest, Grimbeard burned down what was left of the City of Tomorrow after the death of his son, and the battle with Thugheart's forces and the dragon protestors.
The Island of Tomorrow, and the neighboring seas are the main setting.