Oracle of Delphi

Up until the end of The Last Olympian, the Oracle was a skeleton or mummy-like creature (as hinted in The Last Olympian), perhaps. In The Titan's Curse, the Oracle leaves the attic of the big house to speak to Zoë Nightshade and tell her a prophecy to rescue Artemis. Many prophecies have been spoken from the Oracle, probably many that won't be remembered.

The past of the Oracle is finally revealed in The Last Olympian. The spirit of the Oracle is supposed to pass to mortals with true sight, also known as the ability to see through the Mist. The last mortal to host the Oracle's spirit was cursed by Hades after the Great Prophecy was recited, and it led to the death of Maria di Angelo, mother of Bianca and Nico di Angelo. Hades cursed the spirit to never be able to move to another human host, not even when the old host died, until the children of Hades, and himself, were accepted by the Gods and Camp Half-Blood.

May Castellan, mother of Luke, tried to be the new host, without knowing of the curse. She was driven insane because of the curse, leading to Luke running away.

After, Rachel Elizabeth Dare was chosen as the new host of the Oracle's spirit after Hades reconciled with his family and lifted the curse, allowing the next Oracle to be chosen at last.