After capturing a dragon, a young viking has to defy previous beliefs of the animal in order to protect his people from a much bigger threat.

Hi EmelieN,

Here's how I understood the story based on your proposed logline:

"To protect his people against a more severe threat associated with a dragon he captured, a young Viking must ignore the superstitions and dangers that comes with the species."

Use the above logline as a template if you think I got your story right and rewrite your logline.
I hope this helps.
Good Luck