Darkness Falling — Helena Dahlgren

Helena Dahlgren is the winner of 2021 Kids' Book Choice Awards, Age 12-18: Best Fantasy World Builder category. 
Step into the final installment of the fantasy trilogy based on the massively popular game, Star Stable. Follow four friends who discover their magic powers and learn that every girl can be a hero.
A storm is coming. Darkness is falling. Will the Soul Riders find their strength in time to save Jorvik?

From cozy stables to stormy nights and magical places, Darkness Falling is a thrilling adventure where the fate of the world lies in the hands of four friends and their horses.
The four Soul Riders, Lisa, Anne, Linda, and Alex, are facing their greatest test yet. Mr. Sands' sinister plans are threatening to destroy life on Jorvik. Only the Soul Riders and their horses have the power to stop him and his Dark Riders. To succeed, the Soul Riders must learn to master the magic. But they are running out of time.