Are the leprechauns in this game? Do they have zombie variants? If not, I have an interesting idea.
So I was thinking about how the leprechauns say "Got your gold!" when this thought popped into my head of an Evil Dead style voice saying "Got your soul!" Then I started thinking about how that might actually work in game. Then I had it: experience debt.
A zombified leprechaun runs up to you, says "Got your soul!", and gives you XP debt that you can eliminate by killing the leprechaun or by working it off the old-fashioned way.