(Have we seen the rod piece yet?)

You can't see the rod piece, but you know it is VERY close!

DC19 Wis save: 18

(The hertilod will burn its 2nd Legendary Resistance!!) The creature moves with supernatural speed, crawling along the ceiling and down the far wall, still 15' away but on the floor and level with you. (Using Legendary action after Vesta's turn: Sprint)

(OOC - If we were to make use of something that removes poisoned would it just come back because we are still in this environment? Or would removing it allow us to overcome it while we are here?)  

(I guess it would depend on how it removes the condition and if it is a lasting effect. It's the foul air you are breathing that is affecting you...)

(Should have just said - Lesser Restoration. Effect is instantaneous and removes the effect. But if you are saying the room continually causes the condition then that probably doesn't work.) 

(Yeah...it would only last one turn. However - y'all could hold your breath! "A creature can hold its breath for a number of minutes equal to 1 plus its Constitution modifier (minimum of 30 seconds) before suffocation begins. When a creature runs out of breath or is choking, it gains 1 Exhaustion level at the end of each of its turns." I will rule that since you are in combat, you can only hold your breath for 1 minute total - which is still 10 rounds without disadvantage!) 

(OOC - Sorry to keep asking... do we just have to hold our breath? Or we have to remove the condition first, then hold our breath?)

(I'll give y'all a fighting chance - first round is at disadvantage because you've been breathing the air already, then up to 1 minute of holding your breath. I will also add in concentration checks to keep holding your breath when you take damage ;-)

Round 1: (Map!)
Hertilod The giant, skinless, serpentine lizard with sharp claws and a gaping maw that drips venom is awakened from its slumber by the magical attempt to constrain it! It scrambles along the top of the heart chamber, squeezing through the larger opening and gets to about 15 feet away and 50 feet above you! It's long, sinuous neck stretches about 10 feet down, acidic saliva dripping down as it sniffs the air. (It has a 10' reach with its head/mouth...)
Vesta (poisoned) points her Wand of Wonder at the beast, hoping it will do something to impede the creature. But the monster somehow resists the magic, then moves with supernatural speed, crawling along the ceiling and down the far wall, still 15' away but on the floor and level with you.
Zyl 
Davis (poisoned)
Vanizi (poisoned)
Lyrena (poisoned)

Zyl can breathe, and with 40ft of movement, she will take the dash action to go west toward the unexplored area! (By the map I don't think she would incur an attack of opportunity)

Vanizi backs up to just at the passageway they went through from the other chamber.  She first casts Hex on the creature, then follows with Eldritch Blast.
(Spell Attack: 25, Damage: 9 force)
(Spell Attack: 26, Damage: 11 force)
(Spell Attack: 13, Damage: 14 force)
(For the first attack that hits, 1 necrotic from Hex) 

Realizing the negative effects of whatever she's breathing in from this place, she holds her breath. 

Round 1 (con't): (Map!)
Hertilod The giant, skinless, serpentine lizard with sharp claws and a gaping maw that drips venom is awakened from its slumber by the magical attempt to constrain it! It scrambles along the top of the heart chamber, squeezing through the larger opening and gets to about 15 feet away and 50 feet above you! It's long, sinuous neck stretches about 10 feet down, acidic saliva dripping down as it sniffs the air. (It has a 10' reach with its head/mouth...)
Vesta (poisoned) points her Wand of Wonder at the beast, hoping it will do something to impede the creature. But the monster somehow resists the magic, then moves with supernatural speed, crawling along the ceiling and down the far wall, still 15' away but on the floor and level with you.
Zyl moves through the narrow 'valve' to reach the opening to the far west chamber - a 40' x 45' roughly circular area with a 10'-wide petrified artery that leads from this chamber. However, the artery is filled with rocks or stones (or perhaps petrified cholesterol!), blocking that exit from the chamber. The air here is very humid, and black sludge that shimmers like an oil slick is pooled on the floor.
Davis (poisoned)
Vanizi (poisoned) hexes the monster, then...(? the post didn't show what you did =(
Lyrena (poisoned)

(Zyl dashed, as her action, so she should have more movement left. Can she see the rod piece anywhere?)

(OOC - One misplaced ] and most of my post disappeared - although it was still there thankfully when edited!) 

Davis rushes at the beast, on the way, calling on the The Lady's power to speed him on his way, placing himself directly in it's path.

19 dmg: 17

16 dmg: 16

[Forgot about poison so re-rolled with disadvantage, big difference :( Also, haste is concentration, so not sure if you'll allow the concentration roll for holding his breath, but he'll try it anyways.]

Holding Breath Con: 17 / 22  [rolled twice in case poison disadvantage also effects this roll :)]

Zyl can stop at the entrance to the next room (saving her action if you want) and give the space a quick look - no rod piece! (Do you want to take an action, or dredge the black pools of old blood to see if maybe the rod piece lies under the surface?)

The first two of Vanizi's eldritch blasts impact the monster. 

Davis calls upon his goddess and suddenly moves faster than you would expect, hitting the lizard-dragon thing twice!

Round 2: (Map!)
Hertilod attacks the paladin with its claws and maw!
 - Claw Attack 1: 18 Damage: 11
 - Claw Attack 2: 17 Damage: 15
 -  Bite Attack: 32 Piercing Damage: 25 + Poison Damage: 11. If bitten, make a DC 20 Strength saving throw or be swallowed by the hertilod. A swallowed creature has the blinded and restrained conditions, and it has total cover against attacks and other effects outside the hertilod. 
Vesta 
Zyl 
Davis 
Vanizi 
Lyrena? 

Zyl will bonus action rage, then use her action to take the search action.

Perception check: 19

Zyl identifies the sludge as Havock’s blood as she checks the pools for the rod piece, but touching that blood causes her to suffer 12 points of necrotic damage.

The rod piece is not in the pools...

Summoning as much fiery magic as she can, Vesta unleashes a spell of Immolation upon the monster! (Dex Save 19 or take 31 fire damage or half on a successful save)

DC 19 Dex save: 15

The hertilod will use its final legendary resistance to withstand the full brunt of the spell. (Man - it gets advantage on saves against magic, and the Dex save is +10, and it STILL failed the save!!) =O

Davis quickly disappears into the beasts' mouth, swallowed!

Str Save: 17 [used a luck point]