Alfheim no Kishi Ballad: Tsugumi no Mori

After the incident in Harrisburg, Altodias seemed to have regained its peace. However, the beasts have mutated and are causing damage to the people. Robin, who has been treating the beasts in the old castle of Altodias, takes a certain beastmaster's child as his apprentice.

(Source: Akita Shoten, translated)