One of the living hiding in The Oracle's Abode told you of a relic that could help them survive by purifying the tainted water. She directed you to return to The Shining Cove and look for a Sapphire Nagasa shrine hidden in the cliff wall.
Upon investigating the shrine that the survivor told you about, you see something that could be the relic, but debris obstructs the way to it. You then took a Time Rift in an attempt to get to the relic, but it was no longer there in that time.
After investigating the area, you defeated a Void creature that blocked your way and retrieved the relic. Once returned to The Oracle's Abode, the Shrine Maiden was able to use the relic to purify their water.