Erric, you are 10 feet into the tunnel. There is another 5-feet-wide tunnel directly across the room the hill is in. The hill is currently wider than the tunnels, but you're not sure if it will attempt to squeeze into the far tunnel. What do you want to do to keep it from possibly moving into the far tunnel?

Ah, didn't realize. Erric would'nt cast that. He'll fire his hand crossbow at an un-Aseid-looking part of the mound. 

Attack: 12 Damage: 7

And inspire Maldorn with a d8 with his bonus action. "I tried to make it laugh, but maybe shooting sticks at it is better!"

While Erric focusing his efforts on the mound of vegetation, Maldorn turns his bow to the mobile fungus. He shoots two arrows at it, making sure to mark the creature for further hunting (Favored Foe).

-> Longbow + Favored Foe: Attack: 19 Damage: 12  

-> Longbow : Attack: 25 Damage: 10

-> Arrows shot: 4

Maldorn thanks Erric for the inspiring words by letting him know another smaller enemy is approaching from an angle the halfling cannot see at the moment. Hopefully, the old ranger's attacks made clear the direction where this second enemy is coming from. "Back up, Erric. Back up." Maldorn then goes deeper into the tunnel, trying to avoid letting the mushroom get within melee range of him.

Erric expertly fires a bolt at the hill. It doesn't look like it went very far in, though. Maybe it hit something hard on the inside?

Maldorn fires two arrows. The first tears a big hole in the thin flesh of the sentient mushroom, causing it to sort of deflate. The second finishes the job, ripping the top half clean off. Both pieces flutter to the ground and become indiscernible from the other various pieces of vegetation on the floor.

Jambiya, you're up. The hill is still very still. Aseid is still inside. The moving fungus as been defeated.

Jambiya sticks to her (finger) guns, shooting bolts of magic at the stationary mound.

Move: no, she likes where she is.
Action: EB at the mound. 20 to hit, for 10 dmg, and 24 to hit, for 4 dmg

NOTE: Not sure what's going on. All of the rolls have changed, yet it isn't complaining about altered rolls. But the only one I can't really account for is the amount of damage Aseid would take if he fails the CON save, so I guess we'll have to go with what is showing now. The two hits against Maldorn were 20s, and the total damage was 11 and 17.

Jambiya shoots too solid hits at the stationary mound. If it wasn't made of plants, it would look almost bloodied.

Just remembered that I'm supposed to be rolling WIS saves at advantage every time the mound takes damage. So, that's four saves at advantage: 13 13 17 6 — OK, it saved.

Aseid, roll a DC 14 Constitution saving throw or take 11 bludgeoning damage.

The mound moves toward Maldorn and attempts to slam him twice. (Attack: 19 Damage: 15; Attack: 20 Damage: 16) The first hit knocks out the old man. The second sends him spiraling dangerously close to death.

Maldorn has lost two of his three death saves. Technically, I should have rolled the damage as a crit because the mound is within five feet of Maldorn, but nothing would change as the maximum extra damage would be 16, which is less than Maldorn's max HP (max total of 33), which means it wouldn't be enough to insta-kill him.

Aseid, you're up! DC 14 to escape.

(Just disregard those changes, the script here is acting crazy lately)

Aseid DC 14 Constitution saving throw: 22

Aseid was not dead just yet and could fight too. But first he tried to free himself again:

Athletics DC 14: 9

At this point he gave up attemts to free himself for good and could only hope to live long enough to cut the monster from inside.

BA: Second Wind for 9

“Maldorn!” Jambiya shouts (out of turn).

It seems Aseid's body is expending all of its energy to protect itself from being bludgeoned to death and can't find the strength to break free.

Round 5

Erric, you're up! Maldorn, you're next. Then, Jambiya.

You've got one friend who you haven't seen nor heard since the hill seemed to absorb him. And another was just knocked unconscious and then slammed again ruthlessly. Your magic, which seemed to have been holding the mound still, has dissipated against your will.

Erric immediately echoes Jambiya and shouts to his old friend: "Big Spoon!" and casts healing word for 7 hp to the ranger.

(Doing healing word because mantle  of inspiration requires targets can see me, and Maldorn, nearly dying, can't see Erric I'm guessing.)

He then rushes forward and tries to jump on the mound to distract it, drawing his rapier and tries to cut Aseid a way out.

Attack: 20 Damage: 11

Halfling reroll 1s: 18

(OOC: Hmm... I did specify that Maldorn backed up into the tunnel. Did the mound follow? If so, does the mound completely block the tunnel?)

Maldorn 'Big Spoon' Whitehare is completely battered. He never saw what hit him after he took the walking mushroom down, though he has a hunch. Standing back up with difficulty, the ranger deduces he owes his life to Erric once again and let's out a pained single "Thanks..."

He finds a glass jar within his pack and swallows a dollop of magical ointment in his desperation to survive. Then, he backs up even farther into the tunnel (15ft after spending half movement to get back up.

-> Ointment roll to recover HP: 10

You sure did, Lerus. I completely missed (and/or forgot) you had that part at the end of your post. So, the mound would have followed, moved into the tunnel (completely filling its width and height) and aimed those two attacks at Erric. So, that's 28 damage to Erric. He's still up and no one has lost any death saves! How far back did you move, Maldorn? Before you moved, you were five feet in, and Erric was 10.

So, Aseid's turn doesn't need any alterations because nothing that was retconned affects him. We're back to Erric. Restart Round 5!

So you know, Jambiya, you're still in the room, but the mound has filled the tunnel (and is spilling out into the room), but you can have no visual of Erric or Maldorn with the mound plugging the hole.

“Maldorn!” Jambiya does not yell. ;)

Maldorn was hoping Erric would move along with him, so he would try to keep pace with the halfling. Let's say 25ft.

If Maldorn never went down, Would Erric have cast Healing Word on him? It is Okay if not. Perhaps Erric could use that spell himself after the hit.

Erric goes before you, so you don't know if he's going to cast that spell on you. As he just took quite a bit of damage, he might heal himself instead. 25 feet back, you have left the smaller tunnel and are five feet into the larger horizontal tunnel with the standing water. You're standing in the water.

Erric nearly falls from this attack, and will cast dissonant whispers  at 3rd level for 15 damage. He also uses mantle of inspiration on himself for 8 temp hp, as well as Maldorn, and Aseid if at all possible. He uses his reaction to move far away from the dangerous creature.

OOC: Okay, I'm deleting 7 HP from the Healing Word he didn't receive, adding back the 8 HP Maldorn had before accounting for the mound's attack that was not directed at him in the end, and then adding 8 temp HP from the mantle. Maldorn's action for his turn immediately after Erric's is still to heal himself with the ointment. Already being at the other tunnel, he does not use his reaction to move.

(Jambiya is hanging in the ceiling/corner until her turn, but I have lost track of when that is…)

DC 15 WIS save: 9

Erric whispers a discordant melody and feels what passes for the mound's mind recoil from the magic. It immediately "slinks" 20 feet away, stopping half in other smaller tunnel opposite the one Erric is still in and half out.

Erric, do you still want to move away? Also, you can make an opportunity attack if you wish. That would use your reaction. Since the creature moved away, you'd just use your regular movement.

Maldorn, do you still only want to use your ointment and nothing else for your turn?

Erric will definitely move away, clutching his wound. If he can attack with his rapier he will.

Attack: 4 Damage: 1

(That’s +6 and +3, sorry)