In the setting, sorcerers are people that have been possessed by demons; if their willpower is strong, they can command the demon's powers for a time, but, almost inevitably, the demon ends up in control, and, when the host dies, the demon, stronger for having fed on the host's soul, jumps to a new host of it's choosing.

One of the demons in the book jumps to Lady Cattilara, reckoning her to be "weaker" and more easily controlled. Much to the demon's sorrow, however, the lady--however flighty and airheaded she might be most of the time--was deeply in love to the point of obsession with her husband... who just got stabbed in the confused melee that resulted in the death of the demon's prior host. In the space of, oh, about a minute, the "flighty, airheaded" young woman manages to break a full-fledged demon to her will and bids it to keep her beloved husband alive. It does so by stealing life force from his brother. She keeps this up for over three months, to the point where you almost feel sorry for the poor demon...

Anyway, so, I'd like to model this for Shadowrun, as a "WTF was that!?" one-off effect, a sort of reminder to the PCs that "there are greater and more subtle things in the universe than are dreamt of in their philosophy", to mangle the quote.

Thus, my intent is, some Possession spirit, perhaps one summoned by my hougan PC, or an NPC, possesses some metahuman that should be an easy one to possess and control... and instead finds that the metahuman is made of much sterner stuff than they imagined, perhaps to the point of some sort of "innate channeling"--the spirit's in there, but the person's the one calling the shots, much to the spirit's surprise.

Obviously making this a unique effect, but it could be a very interesting--and entertaining--thing to pull on a possession-tradition summoner. "What do you mean, 'she took control of the loa?'"