Oracle of Moonglow

The Oracle of Moonglow is a mysterious voice of wisdom stored the the city's floating library in Ultima IX.

After gaining access to the library, the Oracle revealed to the Avatar how to gain the mantra of Honesty. It also forced the Avatar to admit fault in the Guardians attack upon Britannia before it allowed the Book of Truth to be taken.

Trivia

 * Despite what players might think, the Oracle is NOT that ball of light they see. No, that's just decoration. Using Ethereal Sight one can discover that an ordinary NPC of a man stands inside the light, normally not visible, standing on the pedestal of the Book of Truth. Perhaps the programmers thought no one would return after getting their spellbook back.
 * Forcing the Avatar to incriminate himself for the Guardian's actions is, objectively seen, outright character assasination, since the Guardian has proven to be a being of free will.