The year was 1767.

You were a 19 year old vampire, but had been living for over 2 centuries. You were a member of the Volturi, against your will, obviously. You remembered every grueling detail of your transformation, it was basically burned into your mind because of how many times you thought about it.

The night of your 12th birthday, you had been kidnapped by these monsters that you now call your coven, taken away from your home and kept captive there. They had been experimenting and testing different types of venom on you, not knowing that the venoms had stayed inside of you.

They just thought they didn't work. Eventually, all of the different types of venom created this sort of "super venom". Since the transformation had been postponed, all of it came rushing forward at once, in turn, causing you to have dangerous powers that no other vampire had.