If the creature is moving away in Erric's turn, then Maldorn would feel comfortable moving a bit closer. I had said 25ft earlier, so he could then use the remaining 5ft to move back toward Erric. In terms of his actions, using the ointment takes up Maldorn's main action for his turn. As a bonus action, he orders Sagitta to fly to Erric and coil around his arm for now.

Erric, unfortunately, your arm is shaking because of the attack, and your thrust just bounces along the surface of the mound instead of going in. How far back do you want to move? Maldorn is 20 feet behind you. You can move 25 feet back without the space Maldorn is in being counted as difficult terrain because you're a halfling. You and Maldorn would both be in the standing water of the larger pipe if you do that.

Yes, all the way back. Erric's hurt bad.

OK, Jambiya, the mound stuffed itself into the mouth of the small tunnel you just left (half in/half out) and then "backed out" and is now half in/half out of the tunnel on the other side. You don't see Maldorn, Erric, or Aseid, though you did hear what sounded like feet retreating in the tunnel. It's your turn.

Jambiya climbs down off the ceiling and shouts at the mound. “Hey, ugly! Spit up my friend!” And she shoots two bolts of green and white energy at it.

Move: off the ceiling but no more than 20’ away from it.
Action: shoot it with Eldritch blasts. 18 to hit, for 1 force dmg, and 17 to hit, for 7 force dmg

Reaction: reserved for Hellish Rebuke

Let's clarify. You're 15' off the ground in the corner. Directly below you in the corner of the room (on the floor) is a mass of fungi. Do you want to move down to the floor beside the fungi?

No, she does not, she’s angry at the mound, but not suicidal. She will approach it on the ceiling, instead. She assumes it can’t jump, so she gets closer this way, maybe 10’’ as the crow would fly if it flew such a short distance.

FYI - The mound is 10 feet tall (though probably a little shorter right now because it's condensing to fit in the tunnel), and the room is 15 feet tall, so we'll say you only move 10 feet closer on the ceiling.

Jambiya spider walks across the ceiling. "Hey, ugly! Spit up my friend!" And she shoots two bolts of green and white energy at it. Both bolts land, doing a considerable amount of damage.

The mound stops squeezing into the tunnel and doesn't turn so much as just move back into the room. It moves toward Jambiya and, once below her, attempts to smack her twice. (Attack: 23 Damage: 11; Attack: 11 Damage: 18) It hits her once, but she's able to duck out of the way of the second attack.

Aseid, you're up! At the start of your turn, roll a DC 14 Constitution saving throw or take 8 bludgeoning damage.

As her Reaction, Jambiya will expend her Hellish Rebuke for 25 fire damage or half on a DC 15 Dex Save.

"Take that you overgrown molehill!" she screams at it as green fire lashes out from her.

DC 14 Constitution saving throw: 17

Aseid concentrated all his strength of attacks:

Attack: 8 Damage: 6
Attack: 10 Damage: 14
Attack: 9 Damage: 6

DEX save: 14

Jambiya releases a torrent of magic in retribution, but the creature's strange "hide" seems to deflect some of the power.

Aseid continues to attempt to cut his way out of the mound. He gets a fairly good angle to actually do a little damage with his first thrust, but then something shifts, and his next two attempts don't seem to do any good.

Round 6

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

Erric says, "it looks like you're really soiled yourself," then grimaces in pain at how bad the vicious mockery was for 4 psychic on a failed WIS 15 save. 

Then he casts healing word on himself for 7

Taking only a few steps forward, Maldorn prepares to go on the offensive again: "We have to get that kid out of there!"

Maldorn raises his bow again. He shoots two arrows at the monster.

-> Longbow: Attack: 25 Damage: 6 

-> Longbow : Attack: 20 Damage: 11

-> Arrows shot: 6

WIS save: 15

Erric feels his magic take hold, doing a little psychic damage before talking a little healing into himself.

Not wanting to leave Erric's side, Maldorn stays just outside the smaller tunnel and fires two arrows through the tube at the mound. Both hit. With the second hit, the mound shutters and comes apart, almost melting and filling the whole of the 20x25 floor with various bits of vegetation. Among it is the huddled form of Aseid.

Aseid, you feel your surroundings shrinking and yet expanding at the same time, you go from an upright position to a lying down position as you are pressed down. But almost immediately, the weight of the thing surrounding you is removed as it opens around you, and you find yourself able to breathe once again.

Combat is over.

Dizzy, suffocated and covered in all kind of... let's call it "vegetation" Aseid raised from the floor. He composed himself fast but his voice was still raspy and breathy when he could not help it but to exclaim "What was that thing?". The next question would be "where are we going" - Aseid felt quite lost at themoment - but it had to wait. He looked around suspiciously, ready for any shadow to come alive. "Is everyone alive?"

"That was terrible! I don't want to sing about this," Erric says, marveling at the state of his wounds. "I'm alive. Barely. What happened? How do these things grow? What is this place?"

Maldorn joins those in the middle of the chamber: "Stay away from the fungi on the corners of the room. One of them detached itself and attacked me when I came close enough." He points at whatever is left of the one mushroom he killed.

Turning to Aseid more specifically, he says: "Are you in need of some immediate healing? I still have a dollop of that magical ointment I mentioned earlier. It works wonderfully."

The old ranger will then carefully collect some of his spent arrows. Only three of them seem to be in working condition.

Jambiya will move across the ceiling to gather in the center of the room with the others, only upside down. “Aseid! Thank the good green oaks you’re alive! We need to get out of here, I don’t think this is worth it.”

"I will be fine, it's just bruises." It was a bit more than that but he was not dying. "Miss Jambiya, I gave my word to your father and can not return without the treasure. Yet, your safety is a bigger concern. If you stay here with others, I can check the further tunnels alone."

The little green girl’s eye flash with anger. “Oh, I’ll be having words with my father, don’t you worry, and don’t give me any of that bull-pie about going in there alone!” She marches across the ceiling and into the further tunnel, trailing angry green sparkles. “Are you all coming!”