Video game reference: The old-fashioned camera that Abigail uses in the film is a reference to the popular "Fatal Frame" (known as "Rei Zero" in it's native Japan, and as "Project Zero" in Europe) series of horror/survival games, upon which the "Stay Alive" concept is partially based.
(at around 1h 1 min) [director's cut only] Although the term "Blood Countess" is used in the movie's synopsis, it is never said within the movie. When Hutch and Abigail are hearing the story of Elizabeth Bathory from the author of the book "The Lady of Blood", the author says, "Thus began her obsession" (speaking of using the blood of the girls as a "Fountain of Youth", as Abigail says), to which Hutch replies, "The Blood Countess".
(at around 16 mins) When the group first starts playing the game you will see "Bellman Games Presents" on the TV screens. This is a junction of the last names of both writers William Brent Bell & Matthew Peterman.
(at around 26 mins) [director's cut only] The code to strip the zombie concubines naked that Swink uses is 'Up, up, down, down, left, right, left, right, B, A.' Which is also the Konami Code, a.k.a. Konami Command and Contra Code.
(at around 18 mins) The movie says Countess Elizabeth Báthory moved to America over 200 years ago. Not only is this false, but Báthory died in 1614, so within the movie's setting she's been dead nearly 400 years.
Although they portray brother and sister of almost the same age in the movie Jimmi Simpson who portrays Phineaus is actually 7 years older then Sophia Bush who portrays his sister October in real life.
(at around 53 mins) October, played by Sophia Bush, says, "Go fuck yourself," to her killer before she is murdered. That line is said by Neal McDonough in 2007's The Hitcher (2007), which stars Bush. Coincidentally, McDonough says, "Go fuck yourself," to his killer before he is murdered.