The showdown with the Spirit of Earth. Each time the boss takes 25+ or a round passes the map progresses, the very land itself fighting against the adventurers.
This and my last phased map are out for free, if you enjoy them consider becoming a patron. Over the next few weeks more phased battle maps will be released for my Patrons including a arcane disaster in a wizards library, a perilous trip on a rickety raft down thrashing rapids, and a battle map which is itself the enemy the party will be fighting.