Crossover HTTYD x ROTG
Summary: The most powerful spirit that exist doesn't know who he is. He wakes up all alone, knowing that he needs to find someone named Toothless.
He is seen by no one, not even other spirits or the guardians.
Many years later, Man in Moon chooses him as a guardian, but even as a threat. Manny makes the guardians believe in the unknown boy so they can get him and teach him how to be a guardian.
When the spirit-boy turns off his humanity, the other guardians needs to find a way to get him back, or otherwise he could be a danger for the children.
Rated PG-13 to be safe.