The Ancient Magus' Bride: The Boy from the West and the Knight of the Blue Storm

Story
By a standing stone, the Wild Hunt, a band of knights, appears in the forest, as it does near the summer solstice every 20 or 30 years. One piece broken off the standing stone becomes a horse, separated from the Hunt. Spriggan takes the horse to Elias. The horse tells Elias, through Ruth, that he has lost his rider. Gabe has moved from London with his parents for his health. Walking through the forest, he finds another piece that has broken off the standing stone. When he polishes it in his treehouse, a boy appears that only he can see. Gabe names him Evan and decides to help Evan get back to wherever he needs to go. Independently, Gabe takes Evan, as Chise and Elias take the horse, to the standing stone, where Evan transforms into a knight. After sunset, Evan blows his horn by the standing stone. The Hunt returns so he can rejoin it. Gabe asks Evan to take him with them. Chise, Elias, and Ruth chase after the Hunt to get Gabe back.

(Source: Anime News Network)