Stealing Petitioners

I have this notion in my head that stealing petitioners away from the upper planes is something that a select few fiends manage to do. They figure out a way to infiltrate the plane in question - no small feat in itself - and then, once they integrate themselves into the daily life of an upper planar burg, they start plying their trade (so to speak). They'd do so through the usual means (temptations and whatnot), but because petitioners are technically already dead, the fiends don't have to wait nearly as long to claim their prize.

To accomplish this, I figure that the fiends would have developed specific counters for each plane. For example, something like Fiend Vile. Essentially, it's a distillation of purity and goodness (with some other mystic/magical wuju involved). For the most part, it's a substance that acts as a poison for fiends. In imbibing a Fiend Vile solution, most drinkers would violently expel the contents of their stomach (or whatever else they may have in place), be sickened or weakened or worse. Some would die outright.

In a very select few however, this causes a temporary change in the fiend's outlook, such that it becomes willing to perform acts of unrequited kindness and good. This allows the fiend in question to move about freely on the plane of Elysium, without encountering the usual protections of the plane. They would still have and be aware of their fiendish nature, and they would still have the drive to do evil, but they could better balance it against the needs of Elysium.

Anyway, I'll put up more thoughts as they occur to me, but I thought I'd share with you guys.