In my opinion, remove the whole 'Deal 1 extra damage against a Darkness Mark.' aspect of the weapon. You're making it TOO directly anti-Darkness, which if considered, would just provide too much of an advantage for Light against Darkness. Not only that, but the text is starting to become crammed with all that description in your card box, so you need to 'give some space' so to speak.
The whole killing of Lycanthropes and Vampires is original enough, so that part can stay (since it would work well with a Darkness nymph or Anti-matter + Liquid Shadow strategies).