Derrikoth swears in Draconic, "Filthy beasts! Disgusting creatures!" as he flails his sword at the nearest spider (If there are 2 spiders within his reach, he'll make one attack at each, applying Unwavering Mark on a hit).

Attack: 18 Damage: 8

Attack: 7 Damage: 4

Lydia casts Spiritual Weapon and summons another hammer.

Attack: 9 Damage: 7

Then she casts Toll the Dead again. DC15 Wisdom Save or take 9 necrotic damage.

The spider hit by Nirzeg and Derrikoth is skewered by the both of them, and collapses to the floor between the two of them. 

// Clambering over the dead spider is considered as moving through difficult terrain, but it doesn't stack with the tunnel environment that is already difficult terrain. So still just 2 feet of movement cost for every 1 foot moved. //

Wis Save: 17

Lydia can't catch a break in this cold and oppressive tunnel. The spiders avoid her spells.

Rasa summons his wildfire spirit in the space between the two spiders further into the tunnel! 

Sparky manifests in a wreath of flames. 

Spider Dex Save 11

Spider Dex Save 12

Fire damage: 8 

Rasa will then disengage and move behind Swiper. 

Sparky then flies between Nirzeg and Derrikoth and hovers over the dead spider carapace. (Sparky automatically takes the dodge)

The two spiders try and bite Sparky as he flies away, but the flaming spirit bird is able to avoid them with ease. 

Opportunity Attack: 10 Damage: 8 piercing

Opportunity Attack: 7 Damage: 5 piercing

// Swiper is up! //

"Gaaahhh, I fookin' hate these things!" Swiper shouts as he loads up another shot to shoot at the closest spider!

Attack: 14 Damage: 8
Sneak attack (if applicable): 10

Swiper's bolt blows through the spider's carapace, killing it. Two down! 

(Dex Save: 22)

The remaining spiders advance over the bodies of their fallen, and attack Nirzeg & Derrikoth once more...

Attack: 7 Damage: 4 piercing, and on a hit, Nirzeg makes a DC 11 CON saving throw

Attack: 6 Damage: 7 piercing, and on a hit, Derrikoth makes a DC 11 CON saving throw

(One more spider saving throw: 15)

The spiders are crowding the tunnel such that those in the back ranks (Derrikoth, Lydia, Swiper, Rasa) can only see the spider in front of Derrikoth clearly. If any of them tried to hit the spider in front of Nirzeg with any ranged attacks, they would risk hitting Nirzeg instead.

// Players up! // 

Lydia attacks the spider on Derrikoth with her hammer and uses toll the dead

Attack: 9 Damage: 10

DC15 Wisdom Save or take 17

"Two down. Few left to go." Nirzeg dared to murmur as blade is drawn back, and shield rises higher in defense. Seizing an opportunity in Zeek's slipping past one spider after the other in a confusing course, Nirzeg again brought a vibrating blade to bear with a bark of "Now crumble you eight legged freak!"

Action: Cast (Cantrip)Vibrating Blade - Attack: 19 Damage: 10 of magical slashing and thunder damage and additional 6 thunder damage for volunteering movement.

Spider Wis Save: 20

Derrikoth go chop chop on spider.

Attack: 14 Damage: 8

Attack: 21 Damage: 8

(Assume dragonman's spider is still up) Swiper keeps trying to take down the spooder bothering the horse lover with his crossbow!

Attack: 18 Damage: 8
Sneak attack: 15

Lydia distracts the spider with her spell, allowing Derrikoth and Swiper to finish it off with a one-two, chop-shoot combo. The dead spiders pile up inside the cavern.

Rasa has Sparky use his flame seed ability, in an attempt to kill off the last spider: 

Attack: 20 Damage: 6

Rasa and Nirzeg's attack finish off the fourth spider, and the frost-covered tunnel is once again quiet. 

The path forks here—you have an option of going left or right. 

To the left! 

After continuing and following the path to the left, the tunnel eventually opens up into a small, frosty cavern. This egg-shaped chamber has a flat floor, which is normal terrain. A single natural column supports a domed ceiling 20 feet overhead.

Curtains of icy webbing stretch from the column to the walls, creating an enclosure that holds a massive white spider. The spider screeches audibly, a horrible sound that is deadened by the heavy frost and ice all around you. It attacks! 

Initiative time! Anyone who rolls higher than 17 can act immediately. 

Initiative: 18(See Discord)

"Oh great. Just perfect." Nirzeg gasps! "Wait! This is perfect! I need a sample of that one. Time to size you up for the competition, Big D. Do not mind too much I hope." He request as went through the motions of rubbing a bit of iron powder betwixt the finger tips, and then blowing some in Derrikoth's direction. The moment they land upon his armor, the Dragonborn doubles in sizes to match the spider.

Meanwhile, Zeek wastes no time rushing away for some reason or another.

Action: Cast (2nd level)Enlarge on Derrikoth.

Nirzeg feels a sense of panic from Zeek.

A moment later, Zeek comes yowling to the backline position. And upon catching at least swipper's attention, lands long enough to hiss back the way they came. "Watch yourselves back there. Zeek don't spook that easily without reason."

4

The big ice spider moves through the webs to attack the now-enlarged Derrikoth, first spitting a web before moving in for a bite.

Web attack: 16 on a hit Derrikoth is restrained in the icy webs. 

 Attack: 10 Damage: 6 piercing, and Derrikoth must make a DC 11 CON saving throw. He takes 5 poison damage on a failed throw, or half as much on a successful throw.

This giant spider radiates some kind of freezing aura. Derrikoth takes 4 cold damage at the beginning of his turn while engaged with her, as well as 2 cold damage at the beginning of his turn from the icy webs. 

Behind Swiper, you see more white spiders rounding the corner, following the fleeing Zeek! 

// Players up. Map is updated! //