The Rot Fiend is a mold that inhabits a skeleton of a humanoid. There are three things that make this monster interesting
- Not Undead*
The creature is a actually a mold colony. Although it appears to be a "yellow encrusted skeleton" it is not animated by dark magic: the mold gives the creature life. To me, this means that turn undead and all of the other tricks adventurers use won't work.
- Spongy Bones
The mold has radically changed (or perhaps eaten?) the bone structure of the skeleton it inhabits. The creature is immune to mundane weapons, and invulnerable to bludgeoning (whether it is magical or not).
- Dry Rot
What starts out a constitution damage and, perhaps, an itchy rash will eventually kill a foolhardy adventurer. Woe to those who fail their constitution checks.
Building a Rot Fiend
The choices are just plain obvious here. Weapon immunity for mundane weapons, and nothing for bludgeoning (a holdover from the 2nd Ed version). The spores were kept to a 5ft radius around the creature, and designed as a long term disease. I've included options depending on how you want the disease to play out.
Remember the rot fiend is just as intelligent as a commoner and it is trained in stealth. Most of these creatures probably hide and wait until they can ambush and infect as many adventurers as possible. Download a pdf of the creature here.
*It's technically classified as undead in the 3.5 remake here, but that doesn't suit my interpretation of the flavor text.