Sarissa/Olivia
I mean that he literally killed every member of his (and my) family. Me included. It happened right before I was brought here from my sister's wedding. Some of the others from my world are from further down the timeline, and I'm not alive there.
How he did it is kind of complicated and you have to understand how my coven works. I come from a coven of witches known as the Gemini. Kind of obviously by our name, we rely heavily on twins in our family line. In order for the coven to survive, there must be a set of twins to "merge" after their 20th birthdays. Basically, you train your entire life for the merge, and when the ritual is happening, whoever is the strongest of the two then "absorbs" the other's power, and the weaker twin dies. Technically, the weaker twin becomes a part of the stronger one, hence calling it a merge. The surviving twin then becomes leader of the coven, and so as long as he or she is alive, the coven survives. My father was the leader of our coven my entire life, and it was up to him to produce a set of twins for our coven and family lines to continue.
Kai was one half of that set of twins; he and my sister Jo. Only he was a sociopath and a siphon which basically means he was a witch, but he didn't have any magic of his own - he had to steal it from others. My parents were worried that Kai would win in the merge, so they had multiple other children in order to have another set of twins to go through the merge instead of Kai and Jo. Several kids later, my twin brother and I were born. When I was five years old, Kai killed the majority of all of my siblings out of retaliation for not getting what he considered his due. The only reason Luke and I survived was because of Jo. She hid us and helped us escape until my family locked Kai in a prison world away from anyone and anything. Fast forward a decade or so, and Kai breaks free of the prison world. Instead of merging with my sister, Luke sacrifices himself and goes through the merge with Kai instead - and loses. Luke is dead, and Kai is now the leader of our coven.
He was locked away in another prison world for a while, but when he escaped (with some help), he'd been given vampire blood. He showed up at Jo's wedding, killed her first, and then killed himself only to awaken as a vampire. But since he technically died, the entire coven was killed that night. And now we're here, and I'm stuck with him because I have nothing to go back to back home.