If you have never been to Freewind Post before, simply head south from The Barrens into Thousand Needles and follow the road. When it forks, take the south-east path until you come to a lift. Up the lift is Freewind Post. Once you get there, talk to Rau Cliffrunner to turn in the quest.
Description
Bring the filled phial to Rau Cliffrunner at the Freewind Post in southeastern Thousand Needles. You can get to the Thousand Needles by taking the Great Lift at the southern edge of the Barrens.
I wouldn't be surprised if you can help us out further.
Completion
Hm, yes, this phial of water will serve our purposes perfectly. Because of the shared ancestry of the dryads and the centaurs, the water of the moonwells will enable us to breach the protection of the eternal flame surrounding the centaur relic.