DESCRIPTION
While travelling, the possessed teacher came across another suitable female host who happened to be in the wrong area at the wrong time. While she wasn't the main target that the succubus desired, she was younger and prettier than its current host. Therefore, while the girl was walking through a dark and secluded area, it made its moves. The demon fused both itself and the possessed teacher into the poor girl, simultaneously possessing her while adding the teacher's physics onto hers. Once the deed was done, the succubus salvaged what remained of the girl's clothes and continued on its way toward its main target.
Edit: Had to do some editting, so I added a few extra pages as compensation