writing-prompt-s:

Years ago, you promised your firstborn to a witch. Since then, despite your best efforts, you can’t seem to get laid. The witch is starting to get pretty pissed.

Tired of your shenanigans, the witch uses magic (and possibly the help of an incubi that owed her a favor) to get you knocked up.
To make sure her ‘payment’ was safe she made you live with her during your pregnancy but soon became fond of your presence, child or not.
When the baby came, you cried, expecting your child to be ripped from your arms and then for you to be thrown out onto the streets.
But instead, the witch said that you would stay, and be her apprentice.