So, we need new obstacle, yes?
and we alredy have zombies. So what if combine those with the tar pit!?
So you can place zombies in the path, enemy can not see that, and when u step on it, zombies emerge, use their black magic & so on. Same as tar pit, but you already have those zombies in the game.
One hidden “obstacle” that spaw zombies would be cool. And player can choose where to put that.