Goblin Slayer Side Story: Year One (2018)

Blurb:
The die is cast. After his sister is brutally murdered during a goblin raid, a young boy swears vengeance upon the creatures who killed not only her but also the rest of his village. Five years later and now a novice adventurer, he is by chance reunited with his childhood friend, another survivor of the massacre. Despite his inexperience, crude battle gear, and low rank, the boy sets off alone on a mission to defeat a nest full of goblins-thus begins the origin story of how he came to be known as Goblin Slayer!

Source: www.anisearch.com/manga/40422