Dear Long Lost Son/Daughter
Hey, this is your dearly departed parent. I have a lot to tell you and, since we’re in an Urban Fantasy story, I thought it best to write it down because the chances of me living long enough for you to get past the “gaa-gaa, goo-goo” stage is pretty limited.
You have just turned 16/18/21/met that super hawt guy and everything has changed! You can now see things you never thought existed. You can do things that were impossible! You have powers and skills and a whole lot of teenaged angst about them!
You’re actually a vampire/dhampire/lost faerie princess/super powerful mage/shaman/god* (Delete as appropriate)!
This must be something of a shock to you since we decided to keep it completely secret all of your life, but we had our reasons. See it’s dangerous to be a (insert shiny magical thing) so we decided the best way to help prepare you was to keep you completely ignorant of everything so one day you will have to fight/run from/conquer the terrible, dangerous evil while also dealing with disbelief, culture shock and one hell of a learning curve
If you’re really lucky, you’ll also be doing this while grieving over my recently deceased body.