UNO: Stack Pack Add-On of 16 cards enhances the classic UNO card game by adding stackable draw penalties that increase the game's intensity. Players can use Stack 1 and Stack 2 cards to force the next player to draw one or two cards, respectively, while losing their turn. When responding to other draw cards (+2, +4), these Stack cards can escalate the penalty, making the next player draw even more cards. Wild Stack 3 cards allow players to choose the next color and impose a draw-three penalty on the following player. The unpredictable Wild Stack Number card dramatically heightens the tension by revealing the top number card from the draw pile to determine how many cards the next player must draw. This add-on adds strategic depth and can create significant turnarounds, keeping gameplay exciting.
—description from the rulebook (adapted)