Final Fantasy X Celestial Mirror

To acquire the Celestial Mirror you must first acquire the Cloudy Mirror and take it to Macalania Woods. To acquire the Cloudy Mirror, Go to Remiem Temple in the Calm Lands. Talk to the Chocobo Trainer on the right hand side who will challenge you to a race. If you win the race, you wil receive the Cloudy Mirror. Then go to Macalania Woods, near the southern part, talk to the mother and son standing near the Save Sphere. The mother will then mention that her husband is supposed to be meeting her here, but hasn't arrived and she's worried. Then go to Macalania woods - campsite (Right Path) and talk to the man on the right, who is of course, her husband. Talk to him, and after explaining the situation to him, return to the woman where you will be thanked, then a Mysterious Glowing Path appears...

Head up the path near the save sphere. At the intersection, head west until you see the boy standing in front of large crystal. After talking to the boy, Tidus will use the Cloudy Mirror on the crystal, then the cloudy mirror will then evolve into the Celestial Mirror. This item is one of the most important items you can get in the game, it is the key to obtaining the Legendary Weapons.