Milennia ago, the celestial champion Aera led an epic holy crusade against the Nine Hells, slaying the overlord of the Lower Realms and ensuring the safety of the Planes from fiendish invasion for millennia to come. To ensure that his strength and wisdom would continue to protect the world, Aera was requested by his patron deity to travel to the material plane and father a child with a mortal. Unfortunately, the celestial found himself unable to complete his task, when he realised that the world he returned to had become almost as evil and corrupt as the fiendish planes he had spent centuries fighting against. Aera was not able to find a brave and righteous enough mortal, nor was he willing to introduce a child of his into what had become such a realm of evil.
To complete his final mission, Aera sealed his celestial essence inside this orb and hid it in such a way that only one strong, faithful and righteous enough to bear celestial heritage would be able to retrieve and use it. The Celestian Orb is extremely difficult to locate, even with magic, and the journey to retrieve it is even more arduous. It can only be activated by one who is wholly good and true of heart, and is destined to become a true champion of righteousness. The Orb will simply fail to function for an unsuitable owner, and will instantly annihilate any truly evil person or creature who attempts to use its power.
Once activated, the Celestian Orb transforms its possessor into a celestial champion by granting them the Half-Celestial template. The character takes on an aura of celestial righteousness, as befitting their new responsibility, and their appearance changes appropriately. The orb disappears immediately upon completion of the transformation, a process which takes ten minutes and gives off an incredible celestial light.
Note to DMs: As a sudden jump in power in this manner is unbalancing, you may find it useful to allow the template to be taken one level at a time, as described in Anger of Angels by Sean K Reynolds, published by Malhavoc Press. Perhaps the transformation is interrupted at the last minute, resulting in the subject gaining only one level now and requiring the rest to be unlocked through a series of quests. Alternatively, Aera may simply wish for the bearer to prove themself over time instead of being given the power all at once.
If unable to obtain this book, you could derive it from its opposite, this half-fiend template class. Alternatively, you could have the artifact transform the character into an aasimar, and later progress to half-celestial by means of this aasimar to half-celestial transition class. Whichever method you chose, note that the character should only be allowed to take the first level when they would normally be able to gain a level, which is likely after they complete the quest required to obtain the artifact.