A Crossover with Stargate SG-1.

The slayer is actually a genetically altered line of human females started by a Goa'uld queen. She started the slayer line in order to keep a series of superior hosts for herself and her children. She's been monitoring the line with the help of a cloaked ship in orbit. She's even started to care about the lives of her girls. But now the body of the host that she's had for a few millennia, the first slayer, is failing her so she decides to inhabit one of her girls. She chooses...

You decide:
Which slayer she inhabits; Buffy, Kendra, Faith, ...