Unicorns are built to perform miracles. It is their destiny.
They don't create the same miracles - unicorns never agree on anything, let alone what's valuable. Some bless forests, mountains or storms. Others, the creature's womb.
All of these miracles can turn into dragons.
Kiera warns River several times. it's easy to think you're invincible when you have the magic of a unicorn. When River finally makes a fatal mistake, it's not him who suffers the consequences, but Kiera. now he must do his best to make things right.
He can either retrieve his unicorn or die trying.