HTTYD: Watching their future

Hiccup is walking to the cove where he is hiding Toothless, after the final dragon training (Where Gothi chose Hiccup to kill the Monstrous Nightmare) when suddenly in front of him appears a portal. Hiccup is sucked inside the portal and then everything goes black.
Where did Hiccup go? And what happens when he wakes up?

Read to find out!!