This coaster is a promo item released in 2012. The coaster depicts a crossbow and the following text on the reverse side:
This coaster allows a Knight or Swordsman to hit a flying monster.
Once per game, play this coaster WITH a Knight card to hit a Flying Monster in the Knight ring or with a Swordsman card to hit a Flying Monster in the Swordsman ring. The total damage is 1 point.
For more information: http://boardgamegeek.com/boardgameexpansion/133814/castle-panic-crossbow-promo