"Thanks again for letting me move into your building. Kathy was driving me nuts." said Buffy, as she finished moving her stuff into her new room. Which Xander had cleaned up after his own room and the main meeting areas. Like the bathrooms, kitchem and living room areas.
"Anything for you, Buff." he answered, not mentioning that he had done it because of her irratic behaviour had her friends worried. It helped to keep an eye on her. He continued, "Good thing the University seemed fine with the switch. Hopefully, I'll be less of a annoyance then Kathy."
Buffy smiled at him as she answered, "Yeah, at least I get my own room here."
Meanwhile...
The building hadn't realize the luck it had gotten. The Warrior Woman, the Slayer, was in a unique state. It seemed that her roommate had been sucking out the Slayer's soul. Weakening her physically, but also mentally and spiritually, as her soul was taken. A void had formed, affecting her behaviour. In time, the soul could heal and return her to normal. At the moment, it had a space which could be... filled.
Which was why the building had wanted her here.
Because while there was a void to fill, the Loci could fill it with the same essence it was made of. Creating an agent of it's will, a powerful one. One which it's caretaker and future owner, given Rich's will, was still attracted to. Allowing the Loci to influence him in the right way. The spirit was already at work, merging so people wouldn't be alerted till the Loci was strong enough.
Which was another benefit of the Slayer being here. As all Slayers are connected to the same Slayer Essence. By the filling of the Void, it could reach out and affect the sumbering Slayer as well, when she was weak in a coma.
So the building waited... and sent it's energy subtly into Buffy.