There seem to be no imperials in or around Namai, but Gosetsu asks you to approach it cautiously.
This quest is available if you have completed the quest A Silence in Three Parts, and have reached level 64.
Starting the Quest
Talk to Gosetsu in Yanxia to begin the quest.
Survey Namai from a Safe Distance
Go southwest to the quest icon on the map and check on the sparkling Destination. There will be a cutscene.
Speak with the Young Villager
Talk to the Namai Youth and there will be a cutscene.
Go north to a valley along the east side of the village. Go through that valley to where the orange arrow is pointing in the minimap. Talk to Gosetsu to proceed.
Afterward, go into the middle of the spring and dive underwater by pressing Ctrl+Spacebar on keyboard or by pressing B on Gamepad. Then swim downward until you can swim northwest through the underwater tunnel. At the end, go upward by holding the jump button until you are on the water's surface. Go to the quest icon on the map and talk to Gosetsu to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to the Liberation Front Guard to accept the next quest: A Glimpse of Madness. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.