The snake hisses angrily at the slam attacks and constricts again.

Attack: 26 Damage: 19

Old rotten limbs crack, chunks fall off, and the mound becomes even more shapeless as Boko beats it mercilessly and the snake restricts itself sharply. But still the horrid thing struggles and thrashes, and its rage continues to be directed toward the fey snake confining it, even as it feels the burn of the imagined flames:

Slam Attack #1 vs snake: 19 Hit: 18 bludgeoning damage.
Slam Attack #2 vs snake: 16 Hit: 7 bludgeoning damage.

Qivys, Peri, and Gerrard may act next.

Peri:

Seeing Boko thwacking away at the aggressive lump, Peri quickly circles behind it, if it has a behind. He opens his mind to the Astral and pulls it into him. Two whip-thin tendrils erupt from his shoulders, striking the lump. He pulls them back, like a pair of snakes coiling to strike, and plunges them deep into the mound, feeling for something vital.

((Peri will try to get opposite Boko to be flanking the mound, but stays at 10' range. Hopefully out of its reach, but who knows?

BA: Summon Arms of the Astral Self (1Ki) -- DC14 DEX save or take 11 force damage

Action: 2 attacks: #1 11 for 6 force damage; #2 22 for 10 force damage. He'll also use 1 Ki for each of those to try to Stun the thing -- DC14 CON saves.))

Gerrard:

Seeing the titanic battle rage on, Gerrard finally reacts to it.  “We are fighting against the plant mound, not the snake, right?” He confirms as he draws his Main Gauche and moves to engage the restrained creature.

Elegant Rapier:  Attack: 18 Damage: 9

Elegant Rapier: Attack: 24 Damage: 7

Main Gauche:  Attack: 28 Damage: 13

Qivys joins in on the attack.

Hit: 25

Damage, if hits:  7

Reroll with advantage:  21

The mound attempts to dodge Peri's astral tendrils, even as it struggles within the badly wounded snake. DC14 Dex Save: 3
His first strike hits. DC14 CON Save: 17 

The mound is stunned. Gerrard and Qivys pile on, hammering it with blow after blow. 

Oceanus piles on, skewering it with his trident:
Trident attack #1: 23. Hit: 10
Trident attack #2: 14. Hit: 3

The mound is falling to pieces, but still it shakes and struggles.

Kei and Boko (and her snake) are up next!

Boko attacks again with her staff and again with one unarmed BA attack, urging the others on the finish the plant and hoping her new snaky friend can hold on.

Attack: 17 Damage: 6

Unarmed attack: 22 Damage: 8

Giant constrictor snake constricts again 

Attack: 15 Damage: 15

Kei 

Urged on by Boko and her success, Kei sets himself and begins (what he hopes) his onslaught.

Another radiant bolt streaks from his hand. At the same time a eerie mist forms at the mounds base. A blackish tentacle springs forth and begins to batter the twisted mass of vines and roots. 

A - Guiding Bolt

BA - Tentacle of the Deep 

Concentration - Phantasmal Force

Boko, the snake, and Kei's arcane bolt finish the thing off. It collapses into a pile of mulch.
(Peri may try to make a Sleight of Hand Survival check to locate and preserve a portion of the root-stem, which Peri has read can slowly regrow into a new mound.)

"Halt!" shouts a small voice from the bare treetops. In the dark, you can make out a small fort or treehouse suspended in a thick cluster of branches, perhaps 40 feet ahead and 30' above the black swamp. Ropes are strung from tree to tree. You can see a well-camouflaged ballista pointed in your direction, but not who is operating it. "Who are you?"

Gerrard:

Stepping out and bowing gracefully, Gerrard says, "Greeting, unknown personage.  I am Gerrard Feldren, lately of Saltmarsh.  May I have the pleasure of knowing whom I am addressing?"

Persuasion:  25

"No. What d'you want?"

Gerrard:

"You challenged us, not the other way around.  We want nothing from a disembodied voice in the trees threatening us with a ballista, other than to be left alone to move along our way.  If, on the other hand, you wish to make yourself known and let us know what you desire from us, we would be willing to consider your request," responds Gerrard, miffed at the knavish response.

Kei 

How do adventurers do it? Kei wonders to himself, his nerves on edge from the Shambling Mound and now an unseen, unheard observer. After a moment of composing himself he speaks. "Now now Gerry. That's no way to speak to our friend here. Come down and introduce yourself little voice". He attempts to coax the newcomer out of its vantage point in his calmest most sincere voice.

Persuasion in Avrae

“I am Boko, a local druid. We mean no harm if you do the same. We were tracking some recent sahuagin intruders in these parts and this shambling mound attacked us. I hope it was not your ally. Is that drawn and are you planning to use it on us?” Boko says, poking her staff into the ground and glancing at the ballista. She makes a shushing, suserrating sound and a calming motion with her hand and the snake comes back to her. She quietly thanks the snake for helping them and asks if she can heal it. It slowly moves into a protective circle around her and she pays it gently, careful not to touch its many wounds. 

Peri:

Peri withdraws the astral projections from the broken plant creature and coils them behind his back. He wonders whether they've just killed the tree dweller's guard dog. He also wonders why it allowed the sahuagin past but stopped them. Could be they're allied, in which case Boko's explanation might not be well-received. He attempts to slip quietly into the trees. (Stealth: 14)

((Peri had maneuvered around the mound, opposite the rest of the party, so hopefully the watchers aren't keeping an eye on him.))

Qivy says nothing, taking a defensive stance, shield in front and axe at the ready. 

"Who do you serve?" demands the high-pitched voice behind the ballista. You can hear other small voices arguing behind it. "Why're you here? What'd'ye want?" continues the first voice.

Peri hides. Boko sees that her fey-conjured snake is quite badly injured (2 hp remaining.)

Gerrard:

”We serve the free people of Saltmarsh and all Keoland.  In addition, we are allies with races of good will both above and below the waves—the Sea Elves, the Lizardfolk, and the Merfolk among others,” says Gerrard.  “As Mistress Boko has stated, we are foes of the Sahuagin, who have driven the Lizardfolk from their home and attacked our ships and scouts.”