The Child will hold the secret of the universe and will grant a single individual unlimited power if he hears her out, but if she tells no one, the world will end. She is also the reason for the falling temperatures in the story and will lead to a second ice age if she fails to tell anyone the secret. The result of telling the secret will determine the fate of mankind for the next ten thousand years. The purpose of the Second Indigo Child serves as the saviour of mankind.
The Prophecy is two thousand years old, provided by an unknown prophet, and also doesn't specify when or where the Child will be born. Only the Oracle of the Orange Clan is truly able to locate her through supernatural means, living for two thousand years and performing human sacrifices every seven years in order to receive visions of the Child.
It isn't until the Oracle possesses a man to murder John Winston as the latest sacrifice that another person is also able to receive the visions, Lucas Kane. Because of this, the Invisibles and Purple Clan use Lucas, and also the reason why the Oracle attempts and eventually succeeds in killing Lucas. The only other person known to receive these visions is Anton Janos, who is currently committed into Bellevue Asylum.
By the end of the story, the Indigo Child is found and reveals the secret to either Lucas, the Oracle or The A.I, before dying. Although this fulfills the Indigo Prophecy, a Second Indigo Child is foretold, an unborn child belonging to Lucas and Carla Valenti, apparently for being affected by the Chroma Source and possibly suggesting the Child will someday possess the same supernatural abilities as her father.