It's a one-time use card...You drop it down and there it sits until you choose to trigger it. Based on how other similar cards in the game work it seems like you'd reveal it at the beginning of the overall combat and not a later round within that combat. This forces Dracula to change up his strategy from the beginning. At least that's how we've played it...but I agree that the wording of "a combat" seems muddy so I could be wrong:
"Play at the start of a combat before you choose a card."
Seems like it should either say "Play at the start of combat..." OR "Play at the start of a combat round..." it's that "a" that's mucking it up.
Regardless of when it is revealed, it is discarded immediately after canceling one escape card (either Mist or Bat), or if you use it to cancel a bite or lethal amount of damage. You could also simply leave it out during the combat as a deterrent to Dracula, which is why it seems like you'd reveal it at the beginning of the overall combat.
Yes it was that pesky "a" that confused us. It says "a combat".
Thanks for the answers...I thought it was a one time use but the more I read it, the more I doubted it.