Trivia: According to the DVD commentary, the entire budget for Underworld was the same as the highway chase scene in The Matrix Reloaded.
Trivia: The plot was supposedly loosely based on Shakespeare's "Romeo and Juliet."
Trivia: Kate Beckinsale plays a vampire and Michael Sheen plays a werewolf, both eternally warring in the film. Kate Beckinsale and Michael Sheen were formerly romantically involved, and in fact they have a daughter from their long-time romance.
Chosen answer: In most of the shots, Lucien takes his coat off when he changes to wolf form, and the blade would be attatched to a mechanism in him coat. Also, the film gives us an example of the blade hurting a vampire: bloodloss. And that kind of weapon could do some serious damage, and as it probably wouldn't kill the vampires, it would almost definitely slow them down.