Hexed Stealth second roll: 18 (minimum 25)
Constitution Save: 17 (Using 1/1 Legendary Resistance to succeed). 

Varan CON save (for his own stinking cloud):  19 

Varan maintains concentration on stinking cloud and again spends 1 sorcery point to ensure his five allies automatically pass their saves.

As his action, he draws his crossbow and looks for some sign of where the enemy is hiding - Perception 22 - but to no avail.

AOO?  On an invisible creature leaving melee?  Ok… assuming disadvantage at least?

AOO Attack: 26 Damage: 9

…

Perception: 29
…

     For his own actions, Mulligan would try to follow the path of the blood spray and such to guesstimate their location and use both actions to “grapple”.  Though he’s really not trying to grab or control her, more trying to locate, pat her down, and try to steal back the urn…

Grapple:  17
Grapple:  12
Sleight of Hand: 29

if you want different rolls just let me know…

Now that Keyleth was back on her feet, Ragnar swings around and charges Niko once more. 

"Agent of chaos, are you? I will put into you the fear of justice once more. Let's spare this one from the grave- a more suitable punishment must be found."

Attack: 20 Damage: 9

Attack: 12 Damage: 5

(Not sure about the condition of my griffon, but if it can act as well it'll use the Help action on Ragnar's first attack so that the disadvantage and advantage cancel out.) 

"You're a slippery one, I'll give you that harlot, perhaps it's time to even out the odds." Avarice says and blankets his side of the room in magical darkness before finding a new position by the wall inside the darkness.

(My apologies - I didn't organize turns well enough and things got slightly jumbled.)

Keyleth manages to barely detect the sound of a boot scraping against the stone floor and lashes out with her sword, feeling it connect!
Wulgar attempts two swipes against the space where he predicts the elusive rogue may be, though each swing is neatly evaded. 
Varan solidifies his control over his spell, though can't quite seem to locate the assassin. 
Mulligan detects the wind whip by him and instinctively lunges out with his blade, barely managing to catch Nikto with a slice. 
Opposed Grapple roll of 19: 30 - unfortunately, 
Ragnar is left with only laughs as he feels his blade just barely parried by the hilt of the rogue's dagger. "I've been on trial more times than I can count, tyrant, and I've more bounties on my head than you've probably ever decreed." 
Meanwhile, Avarice mutters an infernal curse - the lights simultaneously wink from existence in an entire corner of the room as it shrouds in a blanket of pitch-black darkness. 

Wulgar laughs in the dark, “How do you like that?  Hah!  That evens the playing field!  Heh heh heh.  I do some of my best work in the dark!”. You hear the Dragonborn chuckle from somewhere in the blackness.

Ragnar - 25
Mulligan - 23
Varan - 18
Wulgar (Rage) - 17
Avarice (True Seeing, Darkness) - 17
Nikto Nikovo (Invisible) - 13
Keyleth - 9

The Changeling laughs, though the gesture is gurgled with blood. The Changeling doesn't seem to mind. "As do I!"

Two daggers fly through the air - one aimed towards Varan, the other directly at Mulligan. 
Hide check: 31 (minimum 25).
Attack (Varan): 24 Damage: 7. Sneak attack, if advantage: 50. DC18 Constitution save for half of 7 Poison. 
Attack (Mulligan): Attack: 33 Damage: 8. DC18 Constitution save for half of 12 Poison. 

As there's only one enemy, everyone can act. 

Wulgar cannot directly perceive the Changeling, but he knows that she is boxed in and surrounded.  He brings his great axe down in two chops directly in front of him, trying to hit her.  

Great axe swing 1 : Attack: 26 Damage: 17 

Great axe swing 2 : Attack: 16 Damage: 15 

On the first hit, additional necrotic damage is done from Divine Fury : 11 

As a bonus action, you hear a loud battle cry from the Dragonborn, and everyone in the party is infused with divine energy for the next turn, gaining advantage on attack rolls and saving throws.

((Through flanking and Zealous Presence, attacking with normal roll at the changeling, let me know if I need to modify or roll differently.))

Despite the darkness filling the now-acidic room, both of Wulgar's strikes swung down and connected with something solid. The Changeling lets out a blood-gurgled laugh as she falls to the ground - hood slipping from her face as blood pours from her throat. There is no regret nor hate in her eyes as she rapidly bleeds out - only hilarity at some sort of cosmic joke only audible in her mind. 

As the last of the light leaves her eyes, her body slacks - and before anyone can react, her body begins to drain of color and crumble into ash, drying and decomposing within a second. All that remains of the Changeling and her equipment is a small pile of dust on the ground. Similarly, a flash of arcane magic pulses through the air as the flask at her side thrums with energy before rusting and crumbling into the same, grayish ash within mere moments. 

Leaving many of you wounded - and only three flasks left in the chamber; one of which has yet to be opened.

Keyleth sheathes her swords, heart pounding, neck aching. "Hells," she manages.

"Indeed." Avarice says, slumping down by the closest wall looking over his grievous wounds, blood soaking his clothes. "Can I get a breather before we open the last flask." He says weakly, blood sputtering from his mouth.

DC 18 Con Save:  14

     Mulligan was moving to smash the changeling's flask but then it crumbled to dust in on itself and he was left there holding the hammer with nothing to do.  Looking around at the party though, everyone seemed a bit worse off.

     "What kind of timeline we have to make this decision?" Mulligan asks nobody and everyone at once.  "Seems best if we take a rest.  A long one, if possible, just in case we have to fight the next one too..."

     "Though..." He continues thoughtfully and hands the hammer to Wulgar (he is the one who has been opening them, yes?) "Perhaps we need keep flask and hammer together and ready."

     Mulligan then moves to Avarice and looks him over.  "I may be able to help a bit..." he says and slowly, reluctantly and cautiously puts his hands upon the Tiefling.  As he does, Mulligan sings a soft healing song to himself, almost a mumble.  But it is a soothing melody and he repeats it twice.  ((Cure Wounds at 3rd for 14 and 20 ))

     Mulligan then shifts his attentions to Keyleth.  "May I?" he asks, holding his hands out gently to both inspect and heal her wounds.  ((2nd level for 15 and 16 ))

     "I can eek out a little more healing if it's a must, but I do think a nice long rest would do us best..."  ((Have four 1st levels... well two, after he uses 2 on himself.  6 and 8 ))

Wulgar breathes a sigh of relief, coming down out of his rage and he slings the great axe back onto his back.  He takes the hammer from Mulligan and nods to him.  "Aye.  That one was so slippery.  I didn't know they could do that, take hold of the flask and fight us like that.  I will hold strongly onto the next flask and this hammer before opening it.  But I agree with you, perhaps a rest would be good.  I'm ok to continue but you lot look rough."

Wulgar looks up at the ceiling and shouts out, "Is that ok with you?  We need to rest for a bit alright?"

"Thanks Mulligan, I appreciate that." Avarice says with a weak smile as some of his wounds closes. "A long rest seems like an excellent idea though." He adds chuckling but stopping short because of the pain.

Varan emerges from behind his pillar and drops the stinking cloud.  "Excellent work, everyone.  But we cannot allow that to happen again.  Wulgar, while your prowess in battle cannot be denied, perhaps someone with... a more delicate touch and spatial awareness... should handle the next and final bottle?  I'd just as soon not see it stolen and face whatever horror lurks there in combat."

Wulgar says “Fine with me, who wants it?”  He holds up the last flask and the hammer to whoever wishes to take it.  

"Thank you," Keyleth says in response to Mulligan's healing. She turns to Wulgar. "I'll take it."

Silence is the only response to the questions asked as the responsibility of watching the flask and the hammer is placed in Keyleth's hands. 

As the party gears down to rest - some more injured than others, a curious sensation washes over you once more - one you have previously felt while resting. Time suddenly seems to halt as you remain, transfixed, held in place by some static force. You watch as the blood spilled on the floor oxidizes and dries within seconds - stubble appears where clean chins may have been. The sensation fades almost immediately after you react to it happen - leaving you confused, exceptionally thirsty, and - besides everything - fully rested. It comes to you - thinking back on the Changeling’s demise - that whoever created this hellscape is also capable of manipulating time within with surprising ease. 

Feel free to take a Long Rest!