The Mysterious Blue Orb is an object that appears in the School of Dragons expansion packs.
The orb is a large, completely round object. It glows bright blue with darker blue ring patterns around it.
Battle for the Edge
Harald Forkbeard manages to steal the orb from the Hunters and sail away with it.
Return to Dragon Island
While exploring the volcano on Dragon Island, the player, Hiccup, Astrid, and Skulder the Archaeologist discover the Green Death inside. After getting Harald out, who they found earlier, he tells them that he dropped the orb inside when he saw the large dragon.
Thanks to the help of the Eruptodon, the player retrieves the orb and clears a path for the lava flow. Astrid realizes the orb is actually a dragon egg.
At the end, the egg is on the verge of hatching and Hiccup takes it to the Hatchery. While it is not revealed what kind of dragon it is, Hiccup says "This . . . changes everything."