I wanted to offer up an example of this. it is quick so please don’t judge.
You have heard rumors of a band of Lizards folk that are in alliance with a group called the forked tongue has set up camp nearby in Bear Paw marsh. Known for it odd shaped waterways and quicksand bogs. They are said to have a gem called the serpents eye set in an idol to their dark god.
area 1: The warning.
Light trickles through the canopy here, showing the strange green mists coming from the bog. in the middle of this clearing is a statue made of bones and scraps of leather. Made as an idol to the One Without Legs. It is clearly a warning to all that you are coming to an area that you are not welcome in.
I would then put a couple of enemies here that would fit the final theme. probably snakes and lizardfolk.
Path to area 2 -
slowly moving across this path, poking around to make sure you avoid the water and dreaded quicksand of this area you advance deeper into Bear Paw Marsh. The little light that was filtering through the canopy is gone. mixed with the heavy moisture and fog of the marsh.
The Graveyard -
Sinking slowly into the marsh is the remnants of an ancient burial grounds. you guess from the symbols and names it is from the Alarian City of Morning Dew that sank into the Ocean nearby when a Kraken assaulted its walls. This mush of been one of the outlying villages that fell during that dark time.
I would give this back drop and fill with appropriate baddies, skeletons, lizardfolk and of course the themed snakes.
Path to section 3 -
Battered, bruised and tired you continue your advance deeper into the marsh. you must be getting sloppy with your checking of the trail as you foot starts to sink into a soft, warm patch of earth.
quicksand trap.
The Serpents Eye - Boss Room
You step past ancient trees flanking you path. you see an effigy to The Legless One now in front of you. Coiled around the skeleton of a man. Its left eye glows with a red hatred from the nearby fires. Chanting in slow hissing words is a Lizards folk and near him is a seemingly contend serpent. it hisses as you come into the clearing.
Appropriate boss fight.
And the players now have a gem from the Effigy known as the Serpents Eye.
that is a quick adventure that could last a session or two. it literally took longer to type up than to think up