The zombie hunt has no currency and no shop of its own. Everything revolves around attacks on bandit camps and wild animals, and the event box in the left-hand menu shows you two progress bars.
The alliance bar
Every attack by an alliance member on a bandit camp (with a decent army) or on wild animals counts for the whole alliance. The more attacks add up, the higher the resource bonus for all members climbs – up to +100%. The bar shows the current bonus in percent.
Your road to a Halloween castle
The second bar is yours alone. It grows with every raid on a bandit camp in which you take loot and with every attack on wild animals. Once it reaches 100%, a Halloween castle upgrade (1,333 fields) is waiting in your upgrade stock.
The upgrade turns a secondary castle into a Halloween castle: it gets 1,333 construction sites and counts as a special castle with bonuses of its own (Special castles).