Not a lot of time was spent on an origin story here. In this universe, Bruce Wayne starts off as the regular, human Batman that we know and fear. He wakes up one morning to find a woman biting him on the neck, and she's a vampire, and for unexplained reasons Bruce is totally cool with this and trusts her implicitly, because we all know Batman loves strangers who sneak into his home and steal his humanity.
"I'm cool as long as you didn't get red on the sheets."
It turns out Tanya's a vampire who has come to Gotham to ask for Batman's help fighting Dracula, and somehow, by turning him slowly over the course of a month, she has given Bruce all the strength of a vampire and almost none of the weaknesses or blood lust. This doesn't make any sense according to any vampire lore we've ever read, but any sense this book was planning on making gets thrown out the window when Bruce suddenly grows wings.
"Ain't nobody's going with God now, Alfred. It's pure metal all the way."