Of course you know, the PC's end up taking the left hand passage! At the fork they were attacked by subterranean skrag trolls and though they escaped, they were forced off course and the current proved too strong for inexperienced watermen to navigate. In fact the current grew to rapids and finally a waterfall (ala Land of the Lost), depositing them in a lightless cavern, battered and without provisions!
Here's the map I drew for the adventure...
I also wanted to include the stylish DM adventure log...
This is typical of the in-game records I kept for hit points and treasure. Apparently the skrag followed the hapless PC's because they fought them again at encounter area B on the map above. Fortunately for the heroes, they stumbled on the Myconid lair next (cave G)... not sure what happened to the spore zombies from cave F, but they're not on the HP sheet so I must have dropped them. After a few tense moments, they managed to convince the fungoid men that they meant no harm and merely sought a way to return to the surface. The Mushroom King agreed to help the PCs in return for a service - a giant crayfish (cave H) currently haunts a nearby lake and the king wants to expand his territory to include this important cavern. If the heroes defeat the monster, the king will provide a guide to the world above. He provides healing salves that return the PCs to full strength after their encounter with the trolls.
He sends along one of his champions as well...
Melf the Mushroom Mage (3HD)
S - 8
I - 17
W - 10
C - 18
Ch - 7 (to non-fungoids, actually a great guy!)
HP - 18 AC- 7 AL - N
Only possessions are a ragged cloak and a wand of magic missiles (14 charges). Spells are not learned through spellbooks, but innate powers, drawn perhaps from the 'hive-mind' of the colony. He may cast two 1st level MU spells and one 2nd level MU spell. Each is usable only once per day and will be recharged with rest. Melf is possessed with overwhelming curiosity. Driving all of his actions will be the quest for experiences beyond the lightless caves and the protection of his new friends.
There is another bit of unique treasure acquired in the crayfish lair - The Ring of Daomund. Apparently this infamous cave explorer was lost years ago, along with the ring he crafted to aid his adventures. Once per day the bearer of the ring may cast the following spells: Continual Light, Locate Object, Find Traps, and Protection from Evil.
With the heroes successfully defeating the monster, the Mushroom King keeps his word. Their guide to the surface turns out to be none other than Melf. Though he does return to the caves after leading them to the shores of Mirror Lake, the PCs will encounter him again in the far future, so his taste of sunlight does not prove to be the disaster Myconid lore claims...