Goblin Slayer Gaiden: Year One  

Goblin Slayer Side Story: Year One, ゴブリンスレイヤー外伝：イヤーワン

The prequel series tells us about the background of Goblin Slayer, including the events that made him into an adventurer dedicated solely to eliminating all goblins worldwide.

Goblin Slayer Gaiden: Year One has been simulpub in English as Goblin Slayer Side Story: Year One by Yen Press.