Our heroes make it through The Grave only to find themselves in the midst of an episode of The Walking Dead, with party favors.
It usually never ends well if you find a random doorway in the middle of a treacherous swamp. Dungeons and Dragons is no different. At the end of the day, our heroes have to go somewhere and what better place than through a random door as undead approach from all sides?
Maybe next time they should just avoid swamps completely.