Below are the objectives for the Journey Into the Past tale in Ghost of Tsushima’s Iki Island DLC.
Investigate the settlement
Travel to Drowned Man’s Shore to find a house. Approach the person crouched by the house to trigger a cutscene. Inspect the ground near this man to find a set of Mongol tracks.
Investigate the Mongol presence
Follow the tracks to a hut. Inside this hut, inspect the clothing on the floor and the corpse trapped by the wooden board. Next, inspect the altar outside the hut.
Defeat the Mongols
After a brief cutscene introducing Shamans, kill all the enemies.
Interrogate the Mongol Shaman
After the battle, approach the Shaman to trigger a cutscene.
Speak with the boatman to sail for Iki Island
Head outside the village to find the boatman located along Drowned Man’s Shore. Speak with him to trigger a cutscene in which you’ll set sail for Iki Island. After arriving on the island, press forward along the path until you reach a wooden barrier. Use your grapple hook to pull the wood allowing you to exit the tunnel to complete the quest.