Ssej
Feeling the hand of your goddess at your back, you manage to move faster than otherwise possible, and manage to get your attacks in. (Not going to revise actions, so your goddess gave you the speed you needed.)

Yashin
The shatter spell goes off, but the 3 figures seem to be able to resist the affects to some extent. (Through magical resistance they all 3 save vs shatter, so take half damage.)

Yuan-ti Malison
#1 shoots 2 arrows at Brock
Longbow. Ranged Weapon Attack: 6 to hit, range 150/600 ft., one target. Hit: 4 piercing damage plus 6 poison damage
Longbow. Ranged Weapon Attack: 5 to hit, range 150/600 ft., one target. Hit: 8 piercing damage plus 11 poison damage

#2 shoots 2 arrows at Valor
Longbow. Ranged Weapon Attack: 20 to hit, range 150/600 ft., one target. Hit: 4 piercing damage plus 6 poison damage
Longbow. Ranged Weapon Attack: 12 to hit, range 150/600 ft., one target. Hit: 10 piercing damage plus 8 poison damage

Yuan-ti Mind Whisperer
#1 casts a spell, hypnotic pattern
Kerah, Valor, Brock, Yashin, and Ichika need to make a DC13 wis save for the spell.

#2 attacks Ssej
Bite. Melee Weapon Attack: 10 to hit, reach 5 ft., one target. Hit: 5 piercing damage plus 2 poison damage.
Scimitar. Melee Weapon Attack: 18 to hit, reach 5 ft., one target. Hit: 5 slashing damage
If either of the above hit...
Mind Fangs (2/Day). The first time the yuan-ti hits with a melee attack on its turn, it can deal an extra 20 psychic damage to the target.

Yuan-ti Abomination
attacks Ssej
Bite. Melee Weapon Attack: 26 to hit, reach 5 ft., one creature. Hit: 5 piercing damage plus 13 poison damage.
Constrict. Melee Weapon Attack: 25 to hit, reach 10 ft., one target. Hit: 15 bludgeoning damage, and the target is grappled (escape DC 14). Until this grapple ends, the target is restrained, and the yuan-ti can’t constrict another target.
Scimitar. Melee Weapon Attack: 14 to hit, reach 5 ft., one target. Hit: 11 slashing damage.

Yuan-ti Pureblood
#1 moves and shoots at Kerah
Shortbow. Ranged Weapon Attack: 12 to hit, range 80/320 ft., one target. Hit: 2 piercing damage plus 7 poison damage.
#2 moves and shoots at Valor
Shortbow. Ranged Weapon Attack: 10 to hit, range 80/320 ft., one target. Hit: 4 piercing damage plus 11 poison damage.
#3 and #4 move and shoot at Yashin
Shortbow. Ranged Weapon Attack: 15 to hit, range 80/320 ft., one target. Hit: 4 piercing damage plus 8 poison damage.
Shortbow. Ranged Weapon Attack: 19 to hit, range 80/320 ft., one target. Hit: 2 piercing damage plus 7 poison damage.
#5 moves and shoots their shortbow at Brock
Shortbow. Ranged Weapon Attack: 8 to hit, range 80/320 ft., one target. Hit: 7 piercing damage plus 4 poison damage.

Valor is now up. Need saves for Hypnotic pattern for those listed above.

Map

Yashin, I had you on the map there already, not sure why you couldn't see it, so let me know if it continues to be an issue.

Ssej laughs at the feeble attacks from the creatures and continues his onslaught on the priest.

Dagger #1: Attack: 32 Damage: 17 Acid Damage: 1

Dagger #2: Attack: 29 Damage: 12 Acid Damage: 3

 Crit Acid Damage on 1st attack: 1

Stunning Strike on 1st hit: DC 17 Con save or be stunned.

Then as his bonus action he will take a bit out of the priest using Hungry Jaws: Attack: 30 Damage: 12 and Heal 4 points if it hits.

[OOC: Ssej is immune to poison damage.]

Valor

Wisdom Save: 24

(Okay base off what I see only two enemies attacked me. The only successful hit because Brock is currently not wearing any plate armor since he was asleep was the Yuan-ti Malison #1 second attack. Brock being a dwarf is resistant to poison so he takes 3 points for the round down. At this time aid has mostly likely faded so for those who increased there hitpoints they are back to normal now.)

Wisdom Save: 14

Ichika Wisdom Save: 17 ((lol.))

Indomitable: 12 

(Forgot to recharge staff)

14

Yashin Wisdom Save: 24

Yashin ducks as an arrow flies harmlessly in the darkness above, then flinches in pain as another one strikes true.   He regrets not sleeping in his armor now, but prepares for the fight ahead.

Kerah's Wis save: 18.
Also, Kerah is immune to poison damage, so only piercing damage taken.

Valor
Continues to sleep, two arrows passing harmlessly near him, both sticking into the ground, or his pack, whatever is nearby. He is oblivious to the hypnotic pattern as it flashes above him, continuing to sleep undisturbed.

Yuan-ti Priests
#1 casts poison spray targeting Ssej
DC 13 Con save for no damage, or take 10 poison damage.
#2 sends 2 eldritch blast rays at Yashin
To hit: 24 Damage: 12
To hit: 8 Damage: 6

Senna, Kerah, Brock, Ssej, Aio, Yashin, and Ichika are now up. Valor you have woken up now as well. If anyone is caught by Hypnotic pattern, then just post that fact.

Seeing all the arrows flying around, including the one that scraped him, Kerah frowns. "Your weapons are a joke!" He calls as he stands up and raises his sickle. "Try to shoot through this!" He calls as he slashes down with his sickle. For a moment, nothing happens. Then, in a 100' cube centred around him (so 50' in each direction), a strong Downdraft blows. He smiles. "Your arrows are no match for Auril's northern winds!" 

((Then, I realise they'd probably hit Kerah with his low AC even with disadvantage... but whatever.))

Senna stands still in the forest looking around to see her companions fighting on all sides. 

Focussing on those near her she raises both hands in the air seeking the aid of her goddess. Under the light of the moon, grant these champions the power of Selûne! 

As she speaks the words the crystal of her necklace starts to glow blue light brighter and brighter. Senna moves her arms and her palms aim for Ssej and for Ichika as the crystals light dispurses and glows upon these two allies as a coat of moonlight around their skin. 

Senna will then try to remain at the position she is now trying to find some cover behind nearby trees as she notices the wind has picked up around her. 

Movement: Perhaps 5 ft (only minimal to find a tree to hide behind vs ranged attacks from enemies near Ssej)
Action: Use twinned spell metamagic to cast Haste on both Ssej and Ichika (according to the map i am in between those 2 and its 30ft range from me to cast)
Concentration: Now on haste for Ssej and Ichika

Ichika feels the blessing of Selune coruscating around her, she turns to regard the source of the divine magic, smiling, giving Senna a knowing nod, before she charges north, gripping her glaive, marked by Chan Cheng for battle, and unleashes her attacks on the Yuan-ti invaders.

Move: to N22, 10' away from the priest #2

BA: Fighting Spirit

Action:
Attack: 21 Damage: 22 
Attack: 19 Damage: 28 

Action Surge:
Attack: 13 Damage: 23 
Attack: 27 Damage: 25 

Haste Attack:
Attack: 10 Damage: 25 

"What the nine hells," yells Brock as he was waking up. The Dwarf not wearing his plate armor stands up. The sleeping Dwarf had bedded down wearing only something similar to a loincloth for decency. As the Dwarf  picks up his shield he deflects an arrow while another one cuts across his arm causing him to bleed. Blocking the third missile, based on past fights people might expect the cazy Dwarf to go into a rage, but there seems to be a calm anger burning in the eyes of the Dwarf as he shouts loudly , but calmly, "I WILL HAVE ORDER."

The Dwarf casts a subtle spell Hypnotic pattern over the group in the south. A orb of light forming with a deep tolling sound of the bell before bursting into a mighty display of multicolored lights.

https://youtu.be/M-59edbWys0

Haste attack vs Priest: Attack: 19 Damage: 11 Acid Damage: 1

Senna
Adjusts her position as she sends magic into her allies.

Kerah
Uses their powers to cause the wind to blast downward, making it difficult to attack at range.

Brock
Causes another hypnotic pattern to appear, this time to the South. All three of the snake-beings there are caught by it, and are dazed.

Ssej
(Forgot the Priest1 was stunned last round anyway, so good thing you wre immune to poison, as it never actually happened.)
Priest1 goes down, as Ssej takes a bite out of him for good measure.

Need Aio's and Yashin's actions still...

Seeing Ichika attack the Priest, Yashin joins her and lumbers south.  He traces a pattern in the air with his warhammer and green flames erupt around the head of the hammer as he brings it down on PB3.

Attack: 14 Damage: 14  plus 7 fire damage

On a hit  7 fire damage surrounds PB4

Need Aio's action still...

The invisible Aio moves due north to to slash at (PB5) with his scimitar three times before moving past the target back into the cover of the forest.  (8 spaces due North for Aio in total please, no AOO due to Mobile)

Attack 1 (Booming Blade): 30
Damage: 12 plus 8 thunder
Movement Penalty for target: 8 thunder

Attack 2: 30
Damage: 14

Bonus Attack: 30
Damage: 10

Notes:
Greater Invis. & Bladesong active (2nd round)
AC=24
Movement=50ft

Crit damage for Booming Blade:

6 weapon damage plus 4 thunder
Extra movement damage: 11

Aio
Invisible, and not able to provoke opportunity attacks anyway, you move forward, strike down Pureblood #5 with 3 powerful strikes, then move onward to the North, still invisible. (took all three strikes to down the enemy, but they are out of combat.)

Yashin
Strikes down pureblood #3 with a single blow, and flames lick at #4, causing more damage to that one, and causing them to no longer be caught by the hypnotic pattern.

Ichika
Takes out the last of the 2 priests with a series of well-placed strikes. She then steps forward again, striking down the pureblood #4 as well.

Yuan-ti Malison
#1 moves towards the invisible form of Aio. They bite at the invisible warrior...
Bite. Melee Weapon Attack: 12 to hit, reach 5 ft., one creature. Hit: 7 piercing damage plus 2 poison damage.
Bite. Melee Weapon Attack: 21 to hit, reach 5 ft., one creature. Hit: 7 piercing damage plus 3 poison damage.

#2 moves South and shoots at Brock
Longbow. Ranged Weapon Attack: 10 to hit, range 150/600 ft., one target. Hit: 5 piercing damage.
Longbow. Ranged Weapon Attack: 23 to hit, range 150/600 ft., one target. Hit: 7 piercing damage.

Yuan-ti Mind Whisperer
#1 sends two eldritch blasts at Brock To hit: 7 Damage: 5
To hit: 9 Damage: 10

#2 casts a spell on Ssej
charm person
DC 13 wis save or be charmed. You have advantage on the save.

Yuan-ti Abomination
Will wait to see if the above save is made or not. It is unlikely Ssej will fail, but just in case...