Godhand is a Celestial Weapon and it is the most powerful weapon for Rikku in the game.
You must obtain the Celestial Mirror before you can obtain or create any of the Celestial Weapons. Click on the link below for more information on how to obtain the Celestial Mirror:
In order to create the fully powered-up Godhand you will need to obtain the Godhand weapon itself first, then use the 
Mercury Crest and the Mercury Sigil to power it up.
Enter the GODHAND Al Bhed Password into the Airship map screen. Check out the Airship Passwords and Hidden Coordinates section for more information on the different secret areas that can be accessed through the map.
Entering GODHAND will give you access to Mushroom Rock as one of the locations in the list. Travel there and follow the pathway along until you reach the treasure chest.
This treasure chest can only be opened if you have the Celestial Mirror in your inventory. It contains Godhand which has some pretty terrible stats to start off with. Time to obtain the crest and sigil!
The Mercury Crest can be found in Sanubia Desert - West on Bikanel Island. Use the menu screen to make sure you are in the correct area of the desert. There is a whirlpool to the west that contains a treasure chest with the Mercury Crest inside it.
You receive the Mercury Crest for successfully completing the Village of the Cactuars side quest. Check out the Cactuar Village section for more information:
Return to Macalania Woods to the area where you received the Celestial Mirror once you have obtained both the Mercury Crest and the Mercury Sigil. Offer Godhand up twice to fully power it up using both the Crest and Sigil.
Godhand has the following abilities when fully powered up:
Break Damage Limit - Allows Rikku to do more than 9,999 damage in a single attack
Triple Overdrive - Charges Rikku’s Overdrive at triple the speed
Double AP - Allows Rikku to earn double the AP for each battle
Gillionaire - Doubles the amount of gil earned after each battle
