Not liking the proximity of this snake-head at all, Eryndor calls upon his Fae learnings to disappear in a shimmer and reappear (Misty Step) behind Ran.

Seeing Ran's foe up close, he pauses, not in fear, but in haughty disgust. He thenviciously mocks the grotesque Yuan-ti "Lo! A nest of vipers wrapped in a sack of arrogance! Pray tell, which of thee is in command, or dost thou bicker amongst thy own limbs like a headless brood?" 

((DC 15 Wisdom saving throw or take 7 psychic damage and disadvantage on its next attack))

[Mica, Meili and Ran - having had a few moments to process the appearance and behavior of your opponents, and reflect upon your experiences in the jungle and your studies of Volo's book, you recognize that the serpent-armed foe is a Yuan-ti Pit Master, while the fighters all look like Yuan-ti Malison (Type 1), though these are fighting with poison-coated blades.  The snake-headed ones casting spells look like Yuan-ti Priest though they seem to have a more potent selection of spells.

Eryndor - yep... these look like yuan-ti, alright.]

Mica appears as if from thin air beside one of the yuan-ti, hacking at it repeatedly with his axes.  It is wounded, but not felled.

Meili, meanwhile, has more success.  She is able to easily sidestep the oncoming yuan-ti without provoking an attack.  Her first strikes finish off the stunned and helpless yuan-ti and she is able to turn her final flurry of blow against the other, pushing it 5' away from her in the process. 

CON:  13

That surviving yuan-ti - between Meili's attacks and the lingering effects of Nalomanterys' fireball - is stunned now as well.  

WIS (w/adv: Magic Resistance):  24

The pit master ignores Eryndor's taunt.  Its tattoed body slick with rain and blood, it lets out a furious, hissing cry in its unnatural tongue.  The words are incomprehensible, but the priests in the distance make note, turning their eyes on Ran.  Then, with another harsh word, the creature's form shimmers and vanishes into a swirl of dark mist - similar to what just happened to Eryndor.

Ran looks around, swords at the ready, but the yuan-ti is already gone—only for it to reappear thirty feet away, just behind Mica.  The ranger, engaged as he is with one of the malisons, barely has time to register the shift before the pit master closes the last bit of distance and its serpent-limbs lash out in a blur of motion. 

[Remember when we talked about the pro's and con's of flanking as an optional rule?]

• Bite Attack (w/adv):  25  Piercing Damage:  10  Poison Damage:  10  Poison's Disciple (bonus poison damage):  9
• Bite Attack (w/adv):  23  Piercing Damage:  4  Poison Damage:  4

The first strikes Mica in the shoulder, fangs sinking deep into flesh, pumping venom into his veins. The second wraps around his side, another surge of poison coursing into him. Mica feels searing pain, his body convulsing from the sheer potency of the toxins, but [if I'm adding hp correctly] it is not enough to make him fall.

At the same time, the yuan-ti priests standing further away pivot toward Ran, their slitted eyes narrowing in focus.  Raising clawed hands, they chant in the same guttural tongue as their leader.  Then, with twin flicks of their wrists, crackling green energy lances out from their palms.

• Eldritch Blast:  20  Force Damage:  10
• Eldritch Blast:  9  

• Eldritch Blast:  23  Force Damage:  5
• Eldritch Blast:  10

Ran is able to duck and dodge two of the energy beams but the other two hit him like haymakers from a prizefighter.  But he too remains standing.

Round 3:

1. Yuan-ti Leaders 
2. Ran 
3. Danica, Eligos, Nalomanterys  
4. Yuan-ti Fighters  and Snakes  
5. Cormyr Mercenaries  
6. Mica, Meili, Eryndor

Ran grimaces as the blasts of energy almost knock him off his feet.

He looks at the man behind him and frowns. "Who the hell are you?" Ran asks. But doesn't wait for an answer. "We're sitting ducks out here in the open."

Ran starts looking for cover, but sees Mica sandwiched between snake heads and takes off at a run, covering the 30' distance as quickly as he can.

Once he arrives, he takes full advantage of how distracted by Mica the Pit-Master is and slashes out twice at him, with Gambit.

Rather than take the third blow he has time for, he takes a second to draw a deep breath and dig deep, hoping to steel himself against wherever the next attack comes from.

Action: Gambit - Attack: 29 Damage: 13 

Second Attack w/ Gambit - Attack: 25 Damage: 12 

Bonus Action: Second Wind - 14 

Ran pursues the pit master.  His blades find their marks—sinking deep into the pit master’s spine with a sickening crunch.  The yuan-ti lets out a strangled hiss, its body arching in agony as the steel cleaves through scales and bone.  When Ran rips the blades free, and the creature collapses to the mud, twitching, its snake-arms writhing feebly.  A final, rattling breath escapes its fanged maw before it falls still, its lifeless eyes staring blankly into the stormy night.

For a brief moment, the battlefield is silent save for the patter of rain.  Then, a shrill, hissing wail rises from the surviving yuan-ti.  The priests lift their voices to the sky, as twin bolts of lightning rend the heavens.  

Eligos, wet and facing one of the yuan-ti, stammer, "Snakes... too many... we should regroup...".  With a wave of his hands, he begins to cast a spell even as he steps back to put a bit of distance between himself and the snakeman.  The yuan-ti uses this opportunity to slash out at Eligos.

• AOO:  17  

It looks as if it is about to cut him down, but the wizard throws up a shield at the last possible moment.  Then he utters a word of power and 4 motes of force dart out striking his attacker.

magic missile:  5  3  2  5

Then he turns and runs west, in the direction of Meili.

The two malisons both turn their attention away from Eligos and Mica, and toward Ran, who cut down their leader.  

• Scimitar Attack:  12
• Scimitar Attack:  13

• Scimitar Attack:  12
• Scimitar Attack:  22  Slashing Damage:  4  Poison Damage:  8

There are a pair of loud 'twang's from overhead.  On the roof of the guardhouse, two of the mercenaries have taken positions and begun firing down on the yuan-ti priests with their heavy crossbows.

• Crossbow:  19  Piercing Damage:  3

• Crossbow:  16  Piercing Damage:  6

Yuan-ti Concentration:  14

[Meili, Mica and Eryndor are up, in any order.]

Eryndor, now frustrated by the yuan-ti spellslingers responds to their attack: "Thou dost wave thy hands and mutter dark curses, yet all I see are overgrown garden snakes pretending at wisdom! Speak not, move not—be as stone, that I may relish the silence!" 

He then thrusts out his lute on his hip as he pours out a series of allegro chords. A shimmering 30 foot cube (DC 15 WIS Save for each of them or Charmed and incapacitated) encompassing the sorcerers and the snakes in between them sparkles for a moment then disappears.

Meili felt herself half-dead but hoped so did the paralyzed snake with the scimitar. She put all her strength - as little as it was - into the next attack:

Unarmed Strike: Attack: 16 Damage: 5
                             Attack: 24 Damage: 10

BA: Attack: 23 Damage: 6

Stunning Strike (1ki) on a hit: CON saving throw (DC 14)

If not stunned  - Crasher - move the snake 5ft and move 40ft in the opposite direction. If the stun worked, stays on the same spot.

Mica has no time to react as the pit master appears behind him, striking twice. Bleeding heavily, Mica moves sluggishly as the poison courses through his veins.  Thankfully, Ran follows the pit master, blades flashing, and cuts him down, allowing Mica to continue to focus on the yuan-ti he has been attacking.  Swaying on his feet, unsure if he could take another hit, he whispers a prayer for speed and strikes out at the yuan-ti with two slashing strokes across its body before moving 50 feet back toward the edge of the trail, trying to get out of reach of the snakemen.

Attack: 26 Damage: 12 plus 6 additional force damage.

Attack: 16 Damage: 6.

.

Ran barely has time to breathe a sigh of relief as the pit master collapses when the other two yuan-ti turn on him. But he has time to react to one of the missed attacks and tries to use the momentum of the miss to quickly strike back at the yuan-ti (the one Mica didn't attack).

Reaction - Riposte w/ Gambit - Attack: 20 Damage: 11  plus 7  additional damage from superiority die

[Hypnotic Pattern:  The yuan-ti have magic resistance.  At the same time, they are snakes... and in fiction snakes are supposed to be susceptible to rhythm and entrancement.  So I am going to apply disadvantage on the save.  For the Yuan-ti that will equal a straight roll, with magic resistance cancelling out the disadvantage.  The swarm of snakes will have full disadvantage.]

• Yuan-ti saves:  3, 17
• Snake saves:  14

One of the yuan-ti priests continues to chant and wave its hands, but the other seems to relax and gazes silently at the air above it.  At the same time, the sphere of darkness vanishes.

CON:  3

Meili continues to lay into the stunned yuan-ti with another series of savage kicks and punches.  It's not clear how much more it can endure.  It remains stunned.

After Ran's riposte and Mica's attacks, both of the yuan-ti fighters near Ran appear to be badly injured, though they show no inclination to retreat.

Responding to seeing him use magic, the uncharmed yuan-ti priest targets the fleeing Eligos with another eldritch blast.

• Blast:  18  Force Damage:  6
• Blast:  25  Force Damage:  22

As he retreats toward Meili and Mica, the blasts both hit Eligos square.  He is knocked off of his feet and crashes to the ground.  He tumbles like a ragdoll into the foliage alongside the trail, inert.  After a moment of smug satisfaction, the priest notices that his counterpart is standing, slack-jawed and idle.

1. Yuan-ti Leaders 
2. Ran 
3. Yuan-ti Fighters  and Snakes  
4. Cormyr Mercenaries  
5. Mica, Meili, Eryndor

Ran shakes his head at the two yuan-ti still standing in front of him.

"You can't be enjoying this, can you mates?" Ran says as he starts in again with his longswords, hoping to drop these two before the wizards decide to target him again.

Action - Attack 1 (to yuan-ti to the south) - w/ Gambit - Attack: 28 Damage: 7  

Attack 2 (against same yuan-ti if standing, switching to the other if not) - w/ Gambit - Attack: 12 Damage: 13 

Bonus Action - (again against same yuan-ti unless dead) - Longsword - Attack: 10 Damage: 11 

Ran cuts down the first of the yuan-ti with Gambit, but the other evades both his remaining strikes.

It slashes twice with its own scimitar in turn.

• Scimitar:  12
• Scimitar:  18  Slashing Damage:  8  Poison Damage:  6

Atop the building, the mercenaries continue to pepper the yuan-ti priests with crossbow bolts.

• Crossbow (with advantage on the entranced yuan-ti):  21  Piercing Damage:  5
• Crossbow:  14  Piercing Damage:  7

"Hah!  Got 'em!" one of the mercenaries crows.  Meanwhile, the enchanted priest shakes off the effects of Hypnotic Pattern.

[Meili, Mica and Eryndor are back up!  And Ran might be down!]

Eryndor assesses the situation quickly. Looking first at the yuan-ti being pummeled by Meili he mocks it: "Most creatures, when struck, at least attempt to avoid further disgrace. But thou? Nay! Thou dost stand still, embracing thy own destruction like a fool dancing into the fire!" ( DC 15 Wis or take 5 points damage)

He then turns to Ran, slinging his lute to his back, and says something that sounds like laughter, or water or is it wind? (Healing Ran for 16 points at 2nd level)

((Dice rolled in log))

Meili barely spared a glance at the man who appeared from the darkness. He seemed to be mocking the enemy - pointless, in her view - but at least he was against the snakes. That was all that mattered right now.

The yuan-ti with the scimitar was still standing and she struck again.

Repeating the sequence:

Unarmed Strike: Attack: 23 Damage: 6
                             Attack: 21 Damage: 4

BA: Attack: 10 Damage: 6

Stunning Strike (1ki) on a hit: CON saving throw (DC 14)

If not stunned  - Crasher - move the snake 5ft and move 40ft in the opposite direction. If the stun worked, stays on the same spot.

Ran curses as the poison clouds his vision and he begins to go down. He takes a knee but then suddenly feels energy surge through his body while at the same time hearing an odd sound. 

“That’s new. Thanks,” Ran says to the world at large. He doesn’t know who did it but it doesn’t feel like the healing he’s received before.

* After dropping Ran, the surviving yuan-ti fighter moves in pursuit of Mica.  He ends up 10' east of the ranger.

Mica pauses, chest heaving as he catches his breath.  He sees a lone yuan-ti following closely behind, and turns back, closing the distance between the two of them and attacking the snakeman.  His axes slash across, targeting the chest, arms and head.  Following the strikes, he dances away thanks to the speed from his prayer and moves 25 feet further away.

Attack: 7 Damage: 4.

Attack: 15 Damage: 5.

Bonus Action Attack: 10 Damage: 5.

Plus 3 colossus slayer damage on a hit. 

.

WIS:  7

As Meili considers the uselessness of the stranger's mocking words, her stunned yuan-ti's eyes roll back in its head after the latest taunt, and it drops to the street, dead.

[Not sure if Meili still wanted to move west 40'?  Moving her on the map but if she want's to stay, then she stayed.]

Mica meets the last yuan-ti heads on, but his axes are unable to scratch his hard scales.  Mica then retreats west.  

[If Ran fell last round, then despite being healed already he hasn't had a chance to get back to his feet yet.]

Seeing that the human who felled the Pit Master is down, the yuan-ti priests turn their attention to the crossbowmen on the roof.  

• Eldritch Blast:  23  Force Damage:  9
• Eldritch Blast:  6

• Eldritch Blast:  16
• Eldritch Blast:  24  Force Damage:  4

1. Yuan-ti Leaders 
2. Ran 
3. Yuan-ti Fighter 
4. Cormyr Mercenaries  
5. Mica, Meili, Eryndor

((Do those bushes and rocks on the eastern side of the map (south of the spellcaster) provide any cover? Say for someone running that way to get closer to melee range without being seen?))

((If the snake died before Meili could punch it again, she would move towards the still standing enemy - like the one near Ran. Then punch!))

[Fair enough.  In that case, Meili has killed the last remaining yuan-ti within melee range of anyone.  

Ran, the two large boulders on either side of the priests are each 15' wide and 15' high.  They would conceal approach from sight, but not sound.  If they were to hear Ran coming, they would only need to move a few steps to put him back in the crosshairs.  But if he could get behind them and be stealthy, he (or someone else) could theoretically approach without getting a faceful of eldritch blasts.]