Lightning Absorption. 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.

Multiattack. 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.

Engulf. 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.

The Forest is my Ally. Once per encounter, this creature summons the roots of nearby trees, vines, or other organic materials to ensnare its enemies. All creatures within 15' of a tree, vines, or other plants must make a DC 14 Strength Saving Throw or be restrained until they spend an action to disentangle or use 1 attack that deals slashing damage to cut free. 

For we are many. Once per encounter, this creature may divide off up to 5 portions of itself. Each portion takes a a 5' by 5' space that becomes difficult terrain. For each portion it divides off it loses 1d10 hit points. At the end of the round, each portion awakens into a [creature]Vine Blight[/creature]. When any of the blights are within 5' of the Shambling Mound, they may spend an action to rejoin the mound. The mound regains 1d10 for each blight that rejoins.

Smelled bad on the Outside. Once per encounter, this creature may use a bonus action to release toxic fumes from mushrooms deep inside the mound. The fumes extend for 30' from the mound and follow the mound as it moves. All creatures who are in the fumes at the start of their turn, or who move into the fumes must succeed on a DC 14 Constitution saving throw or take 13 (2d8+4) poison damage. The fumes dissipate at the start of the mound's next turn.