Long ago there was a wizard that was obsessed with sex. He studied hard and created various spells that enhanced sexual pleasure during intercourse.
His studies eventually lead to a breakthrough. He began creating a spells that would swap any traits, be it mental or physical between 2 targets. He could not, however, figure out why the spell won't work on the caster.
Nevertheless, it gave him an idea. He longed for a partner, but he was very specific. He needed someone he can control or modify. With a goal in mind, he quickly compiled the spells made powerful spell book that gives the user the power to create his or her perfect mate.
The process is as simple:
1. Find a target to become your mate.
2. Use the mind wipe spell to turn your mate into a blank slate only capable of following basic commands.
3.Use various swapping spells from the book to swap body parts, mental traits, memories, fetishes, etc.. from a victim and transfer it to your mate.
4. Rinse and repeat if you get bored or want to make a Harem.
It took him many years to complete but the outcome was fruitful. Centuries pass and the spell book got lost in the sea of time but recently it resurfaced in a library where you are currently located.
It called out to you, wanting to be use again.