@DM
( Two questions. If Najax moves from his position, will it provok a opportunity attacks from the swarms?
Also, if Najax fails his Athletic check next turn, can he lift the altar and hit enemy like a maul? )

Nivi lunges forward, skewing a few spiders in the swarm. She quickly darts back out, the spider carcasses glowing as they quickly overtake them. The bodies combust in a burst of lightning, taking a few more spiders. Dobin slashes at the swarm taking more out of the swarm as they start to crawl onto him. His panther tries to swat at the spiders yet finds too many and too small of a target.

The spiders upon Dobin start to try and bite him.

Attack: 20 Damage: Unable to parse dice roll.

Does Nivi see the attack (critical hit) on Dobin?  If she does, she has a reaction spell she can cast with her free hand (Silvery Barbs).

But the spell description specifies she has to have line of sight on the attack for it to be possible.  I think if Nivi had backed straight out (after going 1 square NE through the door), she might have been able to see it, but the map shows she backed out a little to the northwest, so maybe not.

Reaction "which you take when a creature you can see within 60 feet of yourself succeeds on an attack roll, an ability check, or a saving throw."

Najax was about to cast Bless to him and others. But the sight of the Elf ranger covered by small spiders changed his mind. The Dragonborn. Paladin releases his great sword. He grasps a javelin and swings it at the approaching swarm like a sharp broomstick. 

At the same time, a 10-foot-long yellowish tentacle appears from the portal and whips at the small insects.

@Brand

"Wizard! Fire Boy! Do something! "

He shouts to Brand.

Action: Javelin Attack: 20 Damage: 8

Bonus: Tentacle of the Deeps: Attack: 15 Damage: 11

(Swarm's attack range is actually 0 so no. OoA is only provoked when leaving one's reach, so they only get an attack if you try to leave them when they're on you. And the altar + sword will count as a improvised weapon but will count as a maul in stats.)

(Considering its a swarm attacking, you most likely need a full view of the creature/swarm. So it wouldn't effect them as Nivi had left the room before seeing them strike.)

The spiders are too nimble as the simply scatter away from Najax's strike and his tentacle, slowly encroaching closer upon the paladin. And as the paladin shouts for Brand, he finds himself in the same situation as the ranger. Spiders starting to crawl up onto him, climbing up towards his face. A few of them trying to bite the Genasi.

Attack: 20 Damage: 15

Brand thought of his options. He had spiders, of all things, crawling over him, biting him. Thankfully, between his quick hands swiping them off and the force of his mage armour their bites weren't doing anything. The close quarters was a problem. Too cumbersome for a projectile attack... And his allies too grouped up. He can swerve his flames but how much depended on the power of the spell. Higher spells let him sway more of the effect to protect others from it. So, a higher spell it was, then. 

Muttering a brief incantation as yellow-orange sigils appeared around his hands, he took aim, brought his hands together and unleashed a torrent of flame. He worked the flames, keeping his allies safe.

Brand casts Burning Hands at 2nd level, angled to engulf Swarm 2, Swarm 3 and Ettercap. Using Sculpt Spells to protect his allies from the spell.
DC 15 Dex Save. Fail: 15 Fire Damage. Success: 7 Fire Damage. (Rolled in Game Log)

 (Will need the Panther's save from Dobin.)

Where's the panther? O.o

For the Panther - which attribute is the SAVE for? DEX?

Quote from Cyb3rM1nd >>

 

Brand casts Burning Hands at 2nd level, angled to engulf Swarm 2, Swarm 3 and Ettercap. Using Sculpt Spells to protect his allies from the spell.
DC 15 Dex Save. Fail: 15 Fire Damage. Success: 7 Fire Damage. (Rolled in Game Log)

 
My apologies.

It wasn't my intention to include the panther. He's not on the map and there's no indication where he is. If I had known, I wouldn't have done it.

(Dex Save 15, 15 damage on fail, 7 on success. And just cause of the spell being a 1st level spell, it can only do auto save for two targets, which I assume would be the players, Dobin and Najax.)

Quote from TikoTee >>

(Dex Save 15, 15 damage on fail, 7 on success. And just cause of the spell being a 1st level spell, it can only do auto save for two targets, which I assume would be the players, Dobin and Najax.)

 
I cast it at 2nd level. I did state this in my post. I can protect 3 targets.

(You're right, was considering the spell level itself and not the spell slot. Couldn't check through PHB at that time but did so once I could. Found out my hunch was wrong.)

Despite the spiders crawling upon him, Brand raises his hands and lets loose a stream of fire from them. singing the swarms and ettercap while the party find the fire goes around them. Another swarm of spiders start climb upon Najax, trying to bite the dragonborn paladin.

Attack: 23 Damage: 21

(Davak is next)

Davak attacks the nearest spider

Attack: 18 - 10 damage

Offhand attack: 12 - 9 damage

Davak swings and cleaves through a few spiders, yet the swarm moves onto him. The giant spider crawls onto the ceiling, keeping a bit of distance from the party as it turns its attention to the fire maker, spitting a web towards Brand.

Web attack: 18, if hit, you are restrained

The swarm(2) moves onto Davak, biting at the dwarf

Attack: 19 Damage: Unable to parse dice roll.

The ettercap moves towards Dobin, going fang first as it follows up with his claws at him.

Bite Attack: 20 Damage: 11, if hit Con Save, DC 11, on fail you are poisoned

Claw Attack: 8 Damage: 10

(To free yourself from the webs, it requires an action and a STR check, DC 12. For others to free you, they'll need to attack the webs.)

(I think it's Nivi's turn?  I know the Ettercap's bite requires a CON save against poison, but I'm pretty sure a 10 missed Dobin so not needed.)

Nivi rolls her eyes.  "Did we really have to fight the damn spider queen's vermin inside their damn lair instead of luring or smoking them out?"

She sighs and moves back into the room 30'  I believe getting to the square just north of Dobin and northeast of the Ettercap.  She disengages as a bonus action (Cunning Action) in case she is leaving an enemy's (swarm 1's?) threatened range to get there.  (Edited to add:  just realized she doesn’t need to do this since the swarm’s reach is 0').

She then attacks the Ettercap with her rapier and Booming Blade while flanking with Dobin's panther for advantage and using the best of 3d20s due to her Elven Accuracy feat.  (If no advantage from flanking, then the help below from her owl happens before her attack to give her advantage).

Nivi to hit Ettercap:  22
Nivi rapier Sneak Attack (plus Booming Blade) damage:  8 + 12 (+5) = 20 piercing (plus 5 more thunder damage if the Ettercap moves voluntarily).

Her owl Familiar, Shiv, flies from her shoulder to flap its wings in the face of the Ettercap, taking the help action, granting advantage to the next attack on the Ettercap, flying back to Nivi's shoulder using owls' Flyby capability to avoid opportunity attack.

Whoever attacks the Ettercap next (e.g. Dobin or his panther) has advantage on their first attack.