Actions

Multi-Attack. The Avatar of Nature's Wrath may make one melee attack and use one other action per turn.
Slam. Melee Weapon Attack, one target, 5ft range, +9 to hit. Hit: 4d8+4 Bludgeoning damage.
Lashing Vines. Melee Weapon Attack, one target, 15ft range, +11 to hit. Hit: 2d12+6 Bludgeoning damage. upon failing a DC 16 Strength Saving Throw the target becomes rooted in place (Break DC 21) becoming restrained, their movement is reduced to 0 and they must spend their Action to attempt an escape at the start of each of their turns. An ally may attack the vines wit a slashing weapon to free the creature instead.
A creature that ends its turn still trapped in the vines takes an additional 1d12 Bludgeoning damage.
Downpour [Recharge 4-6]. Extremely heavy rain begins to fall within a 500ft radius of the Avatar of Nature's Wrath for 1 minute, reducing vision to 30ft., and causing all ranged attacks to be made at Dis-advantage while in this area. If used in its lair, Downpour begins to flood the cavern, completely flooding in 1d4 turns.
Suffocate [Recharge 4-6]. Animating a bubble of water over a target within 60ft's mouth and nose, the Avatar of Nature's Wrath attempts to suffocate a creature, forcing water into its lungs for up to 1 minute. Imposing a DC 16 Constitution Saving throw, the creature starts to drown on a failure, taking 1d10 Cold damage. at the start of a creatures turn they may attempt this Save again, taking an additional 1d10 Cold damage each time they fail. If the target is underwater, all Saves against this effect are made at disadvantage.
Earthen Pillar [Recharge 2-6]. Calling upon the earth to violently respond, the Avatar of Nature's Wrath creates a stone lance that erupts from any point that it can see towards an a target within 5ft of that space. Imposing a Dexterity Save against that creature, it takes 2d10 Piercing Damage on a failed save and half of that on a success.

Lair Actions

At an initiative of 10, Losing initiative ties, the Avatar of Nature's Wrath performs actions in sequence with the round number.  

1. Coalesce. The Avatar of Nature's Wrath begins to gather energy from the region, forcing power into itself. until its next turn the Avatar of Nature's Wrath's AC becomes 20 and it gains an additional +5 to hit.
2. Burst. Releasing the stored energy into its surroundings, each creature within 30ft of the Avatar of Nature's Wrath must pass a DC16 Dexterity Save or take 1d10 Force damage and half that on a success. Creatures that take more than half of this attacks maximum damage are knocked back 15ft.
When used underwater, Burst deals 3d10 Cold damage instead and are stunned for one turn instead of knocked back.
3. Grasping Vines. Bursting from the ground, vines attempt to hamper the Avatar of Nature's Wrath's enemies, imposing a DC 16 Dexterity Saving throw against up to 5 creatures within 60ft. of the Avatar of Nature's Wrath, Becoming grappled and restrained on a success.
4. Sap. drawing strength from its opponents, each creature grappled by either Lashing Vines or Grasping Vines must pass a DC21 Constitution Saving Throw or become Sapped, taking 1d4 Necrotic damage. Until Burst is activated again, character who are sapped reduce their to hit by the amount of damage they took, to a minimum of 1.

A site of natural healing and restoration, an Avatar of Nature's Wrath converts a site such as a waste pit or mining shaft into a round pit in the shape of a bulb.   An Avatar of Nature's Wrath when encountered in it's lair is a CR18 monster.