This gilded goblet grants immortality to the person who drinks from it at least once a week. There are side effects, however, if you were to go a single week without drinking from it, you would turn to dust. If the Font is ever broken, it will re-form and appear somewhere else.
Not much is known about how the Sphinx came into possession of the font, other than that it was at Living Mirage, and that he was aided in his retrieval of it by the demon, Nagi Luna.
Bracken gave up his third horn (that accompanies each unicorn when they reach maturity and allows them to enter their unicorn form) in order for the Font of Immortality to be forged of his actual horn.