. Possessing an idol is great for lengthy Karma drains where the spirit drives sacrifices completely insane for their Karma then sacrificing them to summon a great big Blood spirit plasteel homunculus 
If they don't make it in time (took way too long on the way through the cultists' trapped lair) the girl might be insane, dead, or Mr. Blood Servant of the Great Old Ones has already been sumoned and is ready for clobberin' time. A few awesome tid-bits:
-The blood spirit may go free after the summoner is killed, so there's no easy "geek the mage" solution here.
-That idol could be the shadow spirit's true name, giving the runners one opportunity to banish the thing and make it permanent (this thing is going to be pissed
) is a great way to build tension. Potential future plot device I think possession is way cool, and I think many free spirits would agree with me on that one
