Blood Promise is the fourth installment in the Vampire Academy series and was published on 2009. The world that Mead has conceived revolves around a battle between the 'good vampires', otherwise known as Moroi, and 'bad vampires', known as Strigoi.
Mead was intrigued by and researched Romanian folklore prior to writing the novel and based the series on traditional Romanian folklore on vampires.
The story continues with Rose journeying to Russia to kill her lover, Dimitri Belikov, who was turned into a Strigoi in Shadow Kiss. But when Rose encounters Dimitri, she begins to question everything she has ever been taught about Strigoi.
Meanwhile, in St. Vladimir's Academy, Lissa is struggling to cope with the after-effects of using and abusing the use of Spirit and Rose's departure from her life. The story has been noted by fans to be darker in nature than its predecessor.
The Vampire Academy series has sold over 8 million copies worldwide with Blood Promise making the New York Times Best Seller List, along with Mead's spin-off series, Bloodlines. It has been published in multiple foreign countries and recently Vampire Academy has been made into a movie, produced by Mean Girls director Mark Walters.