Thank you for explaining. I had a feeling I had something wrong. Yes, PC possession should be a more difficult thing to achieve and I think I had conflated NPC infection with the PC's fallout. NPC possession seems important to the scenario to me. I was also considering have parts of the black needle spreading out and hiding in the various apartments to make destroying it a little trickier and forcing the players to expore more of the building. I really do like the scenario.