The Olympus Stone is an object in the Olympus Coliseum from Kingdom Hearts II and Kingdom Hearts II Final Mix. It has the ability to protect people from the weakening effect of the Underworld, which, in terms of gameplay, has the ability to unlock the Drive Gauge. The Olympus Stone also is the keychain token for the Olympia, Hero's Crest, and Mark of a Hero Keyblades.
The curse is first mentioned when Sora, Donald, and Goofy tried to attack Hades to get him to stop sending enemies to Hercules to fight. When Sora asks Hercules if there is a way to fight Hades, Hercules mentions the stone. When Hercules goes up to Mount Olympus to retrieve it he returns saying it has been stolen by a man in a black coat who was aided by Nobodies. It is later revealed that Demyx had stolen it to free his powers from the curse of the Underworld. Sora, Donald, and Goofy defeat his water clones and force him back to The World That Never Was. Sora then picks up the left behind Olympus Stone and activates it. It creates a barrier of golden energy that repels the Underworld's curse.
After the defeat of the Hydra, Sora prepared to hand over the stone, but it lit up, signaling him it was the key to opening a new Gate. Sora unlocked the Keyhole; it created a new path to Disney Castle. The Stone was then handed over to Herc, who then returned it to Zeus.
During the second visit, although it is never mentioned, it is used, as Sora is able to fight in the Underworld and is able to hurt Hades.