Moving out of the sleeping quarters you see that this hall is divided into three areas. Two 30 by 30 foot bastions overlooking the entrance, where arrowslits on the north side let sunlight in. And a huge area 60 by 40 foot central chamber, with three cauldrons at its center. There is a fireplace and a staircase going down on each of the east and west walls (the one cloesest to you was not visible from inside the sleeping quarters.) And the south wall has multiple windows opening into the mountain, it's dark in there and you can't see what's on their other side.

As Lothlién and Rhok move into the room and approach the cauldrons, they notice three large and narrow openings in the floor. Each slit ends right next to a cauldron. Looking around they notice 7 giant spiders lurking in the pillars, some 20 feet away from them. The closest one clicks its pincers and starts to climb down from the ceiling menacingly.

Time to fight!
Spiders initiative: 12

Initiative order:
18 Lothlién
12 Seven giant spiders
10 Rhok
  8 Xarroth
  7 Lia

Lothlién goes first.

Lothlién will move away from the spiders, but trying to keep his eyes on them, shouting to  Rhok: "Watch out!"

If there's any way he can cast a Fireball (radius 20ft) without harming his friends, but catching at least 4 of the spiders, that's what he's gonna do
(15 fire damage, DEX 15 save to take half)

If it would hurt his friends, he'll use Shatter instead, again trying to get as many spiders as he can, with no harm to allies.

(18 thunder dmg, CON 15 to take half)

(OOC lol that's the wors fireball ever)

There are no adventurers between Lothlién and the spiders.
Dex saves against fireball: 21, 18, 19, 18, 21.

Zinged by the flames, the spiders climb down the pillars to the floor level. Two of them manage to close in Rhok and try to bite him:
Attack: 12 Damage: 8
Attack: 8 Damage: 8

The ranger evades the spider's pincers. The other spiders try to launch webs against everyone.
To hit Lothlién: 13
To hit Rhok: 6
To hit Xarroth: 13
To hit Lia: 18

Only Lia gets tangled up in the web, she is now restrained.
Lia shields herself from the webbing.

Initiative order:
18 Lothlién
12 Seven giant spiders (7) (7) (7) (7) (7)
10 Rhok
  8 Xarroth
  7 Lia (restrained, DC 12 strength check to burst out, the web can also be attacked normally)

Rhok goes next.

Rhok will fire at the nearest arachnid "Spiders, it HAD to be spiders!"

First attack: Hand crossbow 1:  Attack: 14 Damage: 7

Colossus slayer damage if spidey is hurt:  6 

Crossbow expert second attack:  Attack: 17 Damage: 9 

Second attack: Hand crossbow:  Attack: 24 Damage: 9 

Xarroth silently approves of Lothlién's tactic to rid the world of the creatures - he would have taken the same approach! However, their now proximity to Rhok such a tactic would be a hazard to him, but fire is surely to be effective. With a snap, the translucent orbs breaks in two in the dragonborn's claws and forms into spheres of flame before being slung towards the closest large arachnids!

Chromatic Orb (1st Level)
Twinned
Orb #1 - Ranged Spell Attack: 24 Damage: 17 (fire)
Orb #2 - Ranged Spell Attack: 14 Damage: 9 (fire)

.

Lia casts shield to block the webs.

Lia forms a magical barrier between her and the spiders, deflecting the webbing that was coming her way.

Three bolts from Rhok are enough to finish off one of the spiders attacking him. Xarroth's flaming orbs badly burn the other spider attacking the ranger and left another one smoking too.

Initiative order:
18 Lothlién
12 Six giant spiders (7) (7) (16) (24)
10 Rhok
  8 Xarroth
  7 Lia

Lia goes next.

Lia steps further away fromt eh spiders,t hen sends a blast of flame at one...

fire bolt

To hit: 19 Damage: 12

Lia's fire bolt finish up the second spider that closed up on Rhok, freeing the ranger from melee attackers.

Initiative order:
18 Lothlién
12 Five giant spiders (7) (7) (16)
10 Rhok
  8 Xarroth
  7 Lia

Lothlién goes next, then the spiders.

Frustrated with his fireball, Lothlién tries another spell to hit multiple targets: this time the simpler, more familliar Shatter

10ft radius, 12 thunder dmg, CON 15 to take half

Con saves against shatter: 4, 8, 7, 13.
Web recharge rolls: 1, 2, 3, 3.

An explosion of noise emanates among the spiders, finishing off the one Xarroth left smoking before. The arachnids advance on the party spreading out one to each member, and try to bite.
Attack: 14 Damage: 11 against Lothlién.
Attack: 10 Damage: 5 against Rhok.
Attack: 6 Damage: 6 against Xarroth.
Attack: 23 Damage: 10 against Lia.

Luck seems to be on the adventurer's side, as no spider manages to pin their prey.

Initiative order:
18 Lothlién
12 Four giant spiders (6) (13) (13)
10 Rhok
  8 Xarroth
  7 Lia

Rhok goes next. Feel free to post your actions in advance.

Rhok will fire at the spider attacking him

First attack:  Hand Crossbow -  Attack: 15 Damage: 5 

Crossbow expert attack:  Attack: 26 Damage: 4 

Colossus slayer if spider was damaged -  7 

Second attack: Hand Crossbow - Attack:  Attack: 26 Damage: 5 

Xarroth growls as the spiders advance. Specks of ice and snow begin to swirl in his claws. As he releases, the magnified effects of sheer cold and frost surround the spiders next to himself and Lia, hoping to affect their attacks!

Frostbite
Twinned
CON Save DC - 15
Damage on fail: 7 

Three bolts at point blank range take the spider out. Its legs curl up in the air after it falls back in front of Rhok.

Con saves against frostbite: 9, 4.

Ice crystals form around two of the spiders, slowing down their reflexes.

Initiative order:
18 Lothlién
12 Three giant spiders (18, disadvantage on next attack) (18, disadvantage on next attack)
10 Rhok
  8 Xarroth
  7 Lia

Lia goes next

Lothlién will try to finnish off any remaining spiders with a Firebolt. 

Attack: 18 Damage: 4

(Sorry for the delay everyone, I didn't realize Lia was up.)

Flame bolt attack:
Attack: 13 Damage: 9

Lia almost hits the spider in front of her, but having to dodge so many legs she misses. Lothlién, on the other hand, manages to hit the beast in front of him.

The spiders retaliate, trying to bite the closest enemy they can reach.
Attack: 11 Damage: Unable to parse dice roll. against Xarroth.
Attack: 16 Damage: 4 against Lia.
Attack: 7 Damage: 4 against Lothlién.

Fangs find Lothlién's arm, injecting venom as thy bite (OOC: take 11 poison damage, make a DC11 Constitution saving throw for half)

Initiative order:
18 Lothlién (7)
12 Three giant spiders (18) (18) (18)
10 Rhok
  8 Xarroth
  7 Lia

Rhok goes next.

Lothlién's CON save: 15

Rhok will fire at the spider in front of Loth

First Attack:  hand crossbow:  Attack: 23 Damage: Unable to parse dice roll. 

Crossbow expert attack:  Attack: 18 Damage: 9 

colossus slayer if its damaged:  8 

Second attack:  hand crossbow:  Attack: 17 Damage: 9 

Rhok fires twice at the spider in front of Lothlién, missing one bolt. The second one finds the arachnid's brain, finishing it off. With his last shot, the ranger takes out the spider in front of Xarroth. Only a single spider remains, engaged in combat with Lia.

Initiative order:
18 Lothlién (13)
12 One giant spider (18)
10 Rhok
  8 Xarroth
  7 Lia

Xarroth goes next.