Whenever the shambling mound is subjected to lightning damage, it takes no damage and regains a number of hit points equal to the lightning damage dealt.
The shambling mound makes two slam attacks. If both attacks hit a Medium or smaller target, the target is grappled (escape DC 14), and the shambling mound uses its Engulf on it.
Slam.Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage.
The shambling mound engulfs a Medium or smaller creature grappled by it. The engulfed target is blinded, restrained, and unable to breathe, and it must succeed on a DC 14 Constitution saving throw at the start of each of the mound’s turns or take 13 (2d8 + 4) bludgeoning damage. If the mound moves, the engulfed target moves with it. The mound can have only one creature engulfed at a time.
Source:Pathfinder Roleplaying Game Bestiary.
Shambling mounds, also called shamblers, appear to be heaps of rotting vegetation. They are actually intelligent, carnivorous plants with a fondness for elf flesh in particular. What serve as a shambler’s brain and sensory organs are located in its upper body. Shambling mounds typically have an 8-foot girth and stand between 6 and 9 feet tall. They weigh about 3,800 pounds.
Shambling mounds are strange creatures, more akin to animate tangles of creeping parasitic vines than single rooted plants. They are omnivorous, able to draw their sustenance from nearly anything, wrapping their creepers around living trees to draw forth the sap, sending rootlets into the soil to absorb raw nutrients, or consuming flesh and bone from crushed prey.
Shamblers are frighteningly stealthy in their native environments. Blending in with the surrounding terrain, they can lie in wait for days on end without moving, waiting patiently for a potential meal. A shambler could be almost anywhere at any time, attacking without warning and not caring whether it leaves any survivors, so long as it acquires its next meal.
Shambling mounds normally lead solitary, nomadic existences in deep forests and fetid swamps, although they can also be found underground living among damp fungal thickets. Disturbing rumors persist, however, of shamblers gathering in strange congregations around great earthen mounds in the depths of marshes and jungles, often during the height of violent electrical storms. Their reasons for doing so are unknown, and many sages have wondered whether there is some obscure and alien purpose at work.
Source: 5srd adaptation of Pathfinder Chronicles: Dungeon Denizens Revisited.
Shambling mounds are themselves unpleasant opponents, but some shambling mounds live in symbiosis with swarms of insects, most often centipedes, skittering along the tendrils and vines that make up the shambler. Typically such a symbiotic swarm does not endanger creatures fighting a shambling mound, as too few of the centipedes run along the creature’s surface to pose much of a threat. However, a target subject to the shambler’s Engulf is subject to the Bites of a centipede swarm. The swarm can’t be attacked separately as long as the shambling mound is alive. If the shambling mound is killed, the centipede swarm boils up from its remains and attacks any creatures nearby. Add +1 to the Challenge of a shambling mound with a symbiotic centipede swarm.