This is a really unique concept, Rodriquez Fruitbat!

I think you should replace "he realizes the true cost of its loyalty" with the story's goal/what the boy does after the unicorn turns on his family.

Example: "A troubled boy finds an ally against his tormentors in a dark unicorn, but when its murderous attentions turn on the boy's family, he tries to _______ (goal of story)." Or “attempts to,” “fights to,” “struggles to,” “sets out to,” “fights,” “battles,” etc.