The Endless Journey is first mentioned in the epilogue in How to Be a Pirate. A few months after his own experience on a sinking ship, Hiccup dreams of the Endless Journey sinking beneath the waves, and the captain - presumably Grimbeard the Ghastly - throws a sword into the air that travels through space and time. Dream-Hiccup grabs it. The sword is presumably the Endeavour.
As mentioned in How to Break a Dragon's Heart, after a sequence of events triggered by Grimbeard's murder of his own son, Grimbeard sailed first to an island to mourn and then into the west on The Endless Journey. Legends claim that he reached America and lived the rest of his days among the Feather-people.
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