The Unicorn and His Girl

From Short Dreams 2021 prompts - an attempt of a short story about a black unicorn that will go something like this:

Black unicorns have long been lost to myth. However, with the verge of peace after centuries of war dependent on the birth of a prized unicorn, the stakes have never been higher. When in the dark of night, a black unicorn is born, the girl who has dedicated her life to caring for the rare beasts must decide in an instant what she will do to save her kingdom.

*Finally came up with a title that works for me, and an homage to C.S. Lewis*
(**Prompt is #7)