Godhand
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. Use the link below for more information on how to obtain the Celestial Mirror:
Celestial Mirror
Celestial Weapons

How to obtain the Celestial Mirror, which you can use to create each of the Celestial Weapons.
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.
Obtaining Godhand
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 you can access 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.
You can only open this treasure chest if you have the Celestial Mirror in your inventory. It contains Godhand, which has some pretty terrible stats to start with. Time to obtain the crest and sigil!

Mercury Crest
You can find the Mercury Crest 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.

Mercury Sigil
You receive the Mercury Crest for completing the Village of the Cactuars side quest. Check out the Cactuar Village section for more information:
Village of the Cactuars
Side Quests

Complete the Cactuar Village side quest to obtain the Mercury Sigil, one of the pieces of Rikku’s Celestial Weapon, Godhand.
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.