This page of our guide to Ghost of Tsushima contains a walkthrough of The Birth of Suffering quest.
How to unlock: Complete a side task - Ghosts in the Fog.
Location: Kushi Grasslands
It is one of the shorter side tasks available in the game. All you have to do is get to the temple and kill all the Mongols. They attacked the monks, who were not able to defend their territory. It's best if you jump into the middle of the battle and protect your allies.
After you defeat all the invaders, you will complete the mission.