Yumog is not so much a monster as a collection of urges, feeding off the fears, memories, and terrors (and yes, bodies) of its past victims. It does not assume a physical, humanoid unless absolutely necessary, a process that can take days to compete. Because its a collection of organisms, it has many, many mouths to feed and no time to chew. As such, Yumog utilizes other creatures that dwell in the cave - rat swarms or goblins or a hydra - to capture and kill larger prey, playing their part in the life cycle so that Yumog, The cave's true apex predator, can feed on their putrid remains. 

Yumog feeds on whatever it can find, its spores growing and popping each time it feeds. when inhaled, the spores become powerful, unrelenting hullucinogenics that alter the brain's perceptions and trick the victim into believing they are anywhere but here. The hallucinations Yumog foists upon its victims lure them deeper into the cave, creating images long-lost loved ones just around the corner, long-buried fears that chase them toward danger or illusory bridges over deep ravines until the victims have delved too deep and fallen too hard are too far broken to put up a fight. These spores can create a collective hallucination affecting an entire party or a singular one, presenting an image of horror one moment or a vision of bliss the next. They pull from the infected's mind but also the memories of every creature Yumog has ever consumed. 

Lair Actions

On initiative count 20 (losing initiative ties), Yumog takes a lair action to cause one of the following effects; 

• A fog rises and increases in density; objects and creatures further than 5 feet away from a creature's vision are heavily obscured, effectively limiting the creature's vision to 5 feet. The fog can be dispersed with a strong gust of wind (10 mph or greater) but returns on initiative count 20 on the following round, unless Yumog begins that turn with 50 hit points or fewer, at which point the fog disperses and doesn't return. Yumog and it's minions are immune to the fog's effects, granting them advantage of attack rolls against creatures more than 5 feet away from them. 

Regional Effects

The region containing Yumog’s lair is heavily covered in mosses, lichens, and various undergrowth, which creates one or more of the following effects:

• Hallucinogenic Spores. The air in Yumog's lair is teeming with spores that cause mass hallucinations and major images Yumog conjures to control its prey. When a creature first enters Yumog's lair, there is no save against the spores. After 24 hours, the create must succeed on a DC25 Wisdom check to determine it hallucinating. When they succeed, the effect ends on the creature. They can repeat this saving throw every 24 hours. Illusions operate as the major image spell (DC 20 Intelligence check to reveal the illusion. DC 25 Wisdom check to find the source). 

If Yumog truly dies, these effects fade over the course of 2d10 days.