(No, Kalisia is not familiar with Yuan-ti.)

Kalisia Wongbe: Round 1

Kalisia grunts due to irritation when the poisoned arrow hits her shoulder. She grabs the protruding part of the arrow roughly and attempts to pull it out. After that, she will fumble with a vial filled with distilled spirits while removing it from her pouch and spill the liquid on the wounded area.

Action: [Tooltip Not Found] [False Life] Temporary Hitpoints: 7

"Who's shooting arrows? Show yourself, coward!"

(Do I roll for constitution saving throw to avoid being poisoned?)

According to the Monster Manual, you take poison damage, but it does not say you gain the condition of being poisoned; so, no constitution check is necessary.

One of the Yuan-ti malisons, the larger, snake-headed creatures, is holding a now empty bow.

Erephyma, your turn.

Erephyma, a Druid of the circle of land, swamp to be specific is familiar with the Yuan-ti although there are many breeds.  

Erephyma round 1

Erephema casts Mel’s Acid Arrow at the Yuan-ti holding the empty bow.

”the Lizardfolk are likely to flee if we can take out the Yuan-ti”

The bow-wielding snake-headed Yuan-ti malison gets a face full of acid before he can get another arrow off, but he gets his second arrow knocked and shot at Kalisia: Attack: 5 Damage: 5.

The other malison gets two arrows shot at Erephyma: Attack: 12 Damage: Unable to parse dice roll. and Attack: 14 Damage: 4 

The lizardfolk coming out of the tunnel are caught off guard and with their hands full. They only have time to put down what they are carrying and get their clubs. 

Top of Round Two: Valek, Kalisia, Erephyma, Yuan-ti, lizardfolk

As my last action in Round 1, I will utilize Action Surge to cast Guiding Bolt at one of the Yuan-ti.

Attack:  14

Damage:  20

For Round 2, since we’re still at distance, Valek will utilize his heavy crossbow.

Attack:  19

Damage: 4

I forget to mention, both my attacks will be directed at the Yuan-ti who got the face full of acid.

The Yuan-ti who shot at you is hurting quite a bit. Kalisia, your turn.

Kalisia Wongbe: Round 2

Kalisia creates a sharp shard of ice by instantly freezing a drop of water from her waterskin. She will fling the frozen dagger at the Yuan-ti that's already injured.

Action: Cast a Spell [Ice Knife]

Attack x Yuan-ti: 16 Damage: 1 Piercing Damage

The shard explodes on a hit or a miss, and deals 10 Cold Damage. The Yuan-ti  and adjacent creatures make Dexterity Saving Throw to avoid it.

The Yuan-ti who shot Kalisia goes down.

Erephyma, your turn again.

Erephyma: Round 2

"Focus on theYuan-ti, I will distract the others."

Upon seeing the one Yuan-ti fall Erephyma turns toward the 5 regular Lizardfolk and casts Darkness toward them, trying to envelop all 5 within the darkness to delay their entry into the battle.

A sphere of darkness surrounds one of the lizardfolk, covering all those around it.

Meanwhile, the other large snake-headed Yuan-ti, drops his bow, pulls out a nasty looking scimitar and charges. The three human looking purebloods also pull out scimitars and charge.

(ooc: @DM…the following presumes we have some time to act before the Yuan-ti close the gap. If not, this will be my action.)

Just as the reptiles begin their charge, Valek quickly turns to to the others, “Everyone, stay within 15 feet of me!”  With that, his shield begins to glow with a soft silvery light, similar to that of a full moon, as a dome of soothing twilight envelopes the party.

(The dome of dim light is some 30 feet in diameter, with Valek at its center, and grants all party members within it 1d6+2 temporary hit points. All enemies within it will need to save or be Frightened.)

Yes, you have time to act before they reach you. What's the name of that spell? It sounds familiar, but I cannot place it.

Kalisia, your turn.

(ooc: The spell is called Twilight Sanctuary.)

Kalisia watches the Yuan-ti drop dead after being struck by shards of ice. With a half smirk, she chants something in deep guttural language as if she were cursing the soul of the dead person. Miraculously, she seems healthier than before since she had gained 2 HP. (Grim Harvest)

Kalisia Wongbe: Round 3

During her turn, Kalisia uses a pinch of sand to cast Sleep while tracing her fingers in the air. The pinch of sand transforms into rose petals and she tosses the petals towards the advancing enemy.

Total HP Affected: 29

One of the pureblood yuan-ti falls to the ground asleep. The other two and the snake-headed malison continue advancing.

Erephyma, your turn.