Ah, I found it. The Dark Wizard Of Donkerk by Alexander Wales (summary from here, andalthough I wrote that too)
The basic concept of the plot is that Omarr and Hirrush, two dark wizards in the medieval kingdom of Donkerk, have acquired an infant boy from a local orphanage and plan to sacrifice him for great power. And, well, neither one of them can go through with it. And so they christen the child Henry and begin raising him in their ways, with the help of a local witch, Adrianna. Complicating things, they decide on one last kidnapping (previously their way of raising money), this time of the royal princess (due to her sex and her older brother, they assume that she will be worth a large ransom, but not enough for a manhunt). The kidnapping is successful, but not only does Henry find himself infatuated with the unexpectedly human princess, but their crime also results in a massive manhunt for black magic (it was always illegal, but largely left alone as long as they restricted their harm to voluntary sacrifice). Further complicating things, it seems that there is a prophecy that the princess will fall prey to a dark wizard, and will be saved by an orphan, an orphan born in a destined place and at a destined time that match up with Henry...
The magic I was thinking of is that of the oathkeepers. (capitalization is consistently lower-case for their name):
