Does anyone have an idea of what would define the "powerful holy magic" needed to change an existing creature into a half-celestial one?
Also, would a DM typically allow for a PC to have the half-celestial template applied to their character in the first place?
I would generally not allow templates on any player characters. I'm hesitant to allow non-core races for my players. Depending on the power level of the game and of the other PCs, templates might be acceptable, but I generally just say no and compensate with increased gear.
As far the the "powerful holy magic" that pretty much seems like code for "GM fiat". Essentially, there isn't a pre-printed spell that can do this for you, but rather some sort of ritual or experience with divine magic from a celestial source that is simply described by the GM.
