Goblin Slayer Side Story: Year One

Goblin Slayer Side Story: Year One (ゴブリンスレイヤー外伝：イヤーワン, Goburin Sureiyā Gaiden: Iyā Wan) is a prequel series to the Goblin Slayer! light novels. The story is written by Kumo Kagyu and illustrations are done by Shingo Adachi. A manga adaptation by Kento Sakaeda began serialization on September 15, 2017 in Young Gangan.

Plot
After his sister is brutally murdered during a goblin raid, a young boy swears vengeance upon the creatures who killed her and 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 lair of goblins--thus begins the origin story of how he came to be known as Goblin Slayer!
 * -Yen Press

Manga
The following chapters were serialized in Monthly Big Gangan and published digitally by Yen Press, but have yet to be collected in a volume.
 * Chapter 13
 * Chapter 14
 * Chapter 15
 * Chapter 16
 * Chapter 17
 * Chapter 17.5
 * Chapter 18
 * Chapter 19
 * Chapter 20
 * Chapter 20.5
 * Chapter 21
 * Chapter 22