Doc's spear flew straight into the darkest recesses of the webbing ... and drew forth an unimaginable howl or rage.

The other heroes advanced, weapons at the ready, as the spider swarm erupted from the webbing in a writhing mass of hairy legs and chittering fangs. Chromir’s great axe swept through the swarm, scattering small bodies but doing little to thin their numbers. Cassian’s longsword struck with precision, and scattered many of the spiders from the swarm,  yet the swarm seemed endless, surging over blades and boots. Chromir almost  fell beneath the tide of spiders, their countless bites bringing the mighty barbarian almost  crashing to his knees.

From deeper within the web-shrouded shadows, a blazing orange glow flared to life. It looked like a smelting furnace was being stoked. The air grew oppressively hot as the hidden beast revealed itself — a monstrous arachnid with eyes that blazed like molten coals. Its mandibles clicked and clacked, dripping with an oily fire. With a guttural hiss, it reared back and spewed a gout of flaming webbing toward Doc. The fiery mass sizzled as it hurtled through the air, narrowly missing the dwarf and leaving a charred mass of webs stuck to the stone wall behind him.

Heat rippled outward, and for a moment, it seemed the very room had become the spider’s hellish domain.

Init - PC [HP]; Conditions & Effects; Actions
23 - Navrine [06/10hp]; Move & Ready Attack
19 - Chromir [07/15hp]; Rage active; Move and Attack swarm, a hit! [2hp, after damage resistance]
--> Later in the round, Chromir was brought to 0hp remaining and he is now Unconscious (which ends his Rage)
17 - Cassian [14/14hp]; Move & Ready Attack
16 - Doc [11/11hp]; Stonecunning active, Move & Attack Inferno Spider, a hit! [9hp], Move

Init - Opponent [hp inflicted]; Conditions & Effects; Actions
16 - Swarm of Spiders [4hp], Enter Chromir's square ...
--> This movement triggers Cassian's Readied attack, a hit! [2hp, after damage resistance]
----> Swarm of Spiders Attacks Chromir, a Critical Hit! [12hp, after damage resistance]
------> This brings Chromir to 0hp remaining

EDIT — the swarm had disadvantage on the attack which took the critical hit off of the table.  Still a hit, but only 12hp now (which gets reduced to 6hp due to Chromir's Rage). Apologies, and thanks to EmileTherese for pointing out the error!
------> This triggers Chromir's Stone's Endurance ... further reducing the damage to 0hp.

 7 - Inferno Spider [9hp], Web Attack vs Doc, a miss! (disadvantaged due to distance)

As a swarm, the swarm of spiders is resistant to Piercing, Slashing, and Bludgeoning damage.

The area is unlit, save for Navrine's torch. The purple area is an area of webbing. This webbed area counts as Difficult Terrain and is Lightly Obscured. It is not very easy to see, but Chromir and the Spider Swarm are occupying the same square.

Chromir sees his life flash before his eyes as the spiders swarm his body. But then his skin hardens like that of his stone giant ancestors, and the spiders fail to pierce his skin. The spiders on his body get removed by his allies, so he turns his attention to the large spider. He grasps one of his 5 javelins from their quiver(? for lack of a better word) on his back and throws it at the spider, before moving 20ft north to be between the spider and Doc while out of the webs.

14 to hit for 8 piercing damage. (and slow gets applied)

Movement: Navrine Darts 5’ southwest around Cassian to try and squash the spiders attacking Chromir. 

Action: Unarmed attack, to hit 18,  damage 8 haved to 4

Bonus Action: Unarmed attack, to hit 12,  damage 8 haved to 4

Cassian's eyes widen as the wave of spiders swarms over Chromir only to be repelled by the Goliath. Now's their chance to get rid of them so they can focus on the big one. Taking a breath he aims another swing at the spiders with his longsword, focusing radiant energy into the blow.

A and BA: Longsword to hit 16, damage 5 slashing (halved to 2) + 9 divine smite and sap

((Doc is at 6hp, not 11 thanks to the trap in the hallway))

Doc doesn't have many options left so moves forward to also hit the swarm with his mace,after which he moves back behind Chromir, knowing he should try not to be a target as the healer.

9 to hit for 9 damage.

Rallying together, the heroes turned their fury upon the spider swarm. Navrine's strikes became a blur of crushing blows and Cassian’s blade — imbued with a divine radiance — carved through the writhing mass.  With every attack, the swarm weakened until, at last, its chittering fell silent, the mass of tiny bodies lying lifeless at their feet.

Chromir ripped a javelin from his pack and hurled it with all his might toward the Inferno Spider. The weapon sailed through the air, striking true and sinking deep into the creature’s abdomen. The spider shrieked, a piercing, fiery wail that rattled the scorched beams of the room. Reeling back, it retaliated with a burst of flaming webbing, the searing strands streaking past Chromir, missing by mere inches.

Init - PC [HP]; Conditions & Effects; Actions
23 - Navrine [06/10hp]; Move & Unarmed Attack vs swarm, a hit! [4hp, after damage resistance]
--> BA: Unarmed Attack vs Swarm, a hit! [4hp, after Damage Resistance] 
19 - Chromir [07/15hp]; Rage active; Move & Attack Inferno Spider, a hit! [8hp w/slow]
17 - Cassian [14/14hp]; Divine Smite vs Swarm, a hit! [2hp slashing & 9hp radiant]
--> This drops the swarm to 0hp remaining
16 - Doc [06/11hp]; Stonecunning active, [Doc's declared action became unworkable. Next round he may declare two actions]

Init - Opponent [hp inflicted]; Conditions & Effects; Actions
16 - Swarm of Spiders [Dead]
7 - Inferno Spider [17hp]; Bloodied; Recharged Web & Web Attack vs Chromir, a miss!, Move

The purple area is an area of webbing. This webbed area counts as Difficult Terrain and is Lightly Obscured. Now that the inferno Spider is aflame, it burns its own webs away as it moves.

Chromir does not want to get stuck in the webbing, and being unable to clear it will throw another javelin. Unfortunately his aim is off and the javelin misses (At least I assume it does with a 9 to hit)

Cassian will follow Chromir's lead, seeing that the spider's aim seems lacking at this range. He will move up and left 5 ft and then throw one of his own javelins with all his might.

Action: Javelin Throw to hit 23, 6 piercing damage (3 if resistant).

Movement: Navrine moves 5’ north then dashes through the webs hopefully arriving just north of the giant monstrosity. "Die you eight-legged-monstrosity!" She yells as she boldly charges into the webs.

Action: dash

Bonus Action: Unarmed attack, to hit 8,  damage 5 (halved to 2)(but pretty sure that does not hit) 

The swarm was resistant to piercing, slashing, and bludgeoning damage because it was a swarm.  No need to halve any damage for the giant spider.

Doc moves forward aswell, standing at the edge of the web readying an attack with his mace.

To hit 17, dmg 6

Three of the heroes positioned themselves at the edge of the dense webbing. Doc stood firm in the center, mace and shield at the ready to protect his comrades if the Inferno Spider dared to close in. Chromir and Cassian each hefted their javelins and hurled them!  Chromir's missile struck stone, splintering harmlessly, but Cassian's found its mark, embedding itself deep in the spider’s thorax with a sickening crunch.

Unwilling to wait, Navrine plunged headlong into the sticky maze of webbing, her movements graceful and determined as she closed the gap. Her fists shot out in a blur, but the Inferno Spider scuttled back just in time, her strikes finding only empty air. The monstrous arachnid seized its moment, rearing up to reveal fangs blazing with unnatural fire. It lunged, sinking its fiery fangs deep into Navrine’s shoulder. She gasped, the flames searing through her body, before crumpling to the ground, unconscious.

The monk's breath became shallow as the fire venom ran its course.  Navrine was in trouble!  Everyone knew that another strike from the monstrous spider could very likely prove to be her last.

Init - PC [HP]; Conditions & Effects; Actions
23 - Navrine [00/10hp]; Move, Dash, BA: Unarmed Strike, a miss!
--> Later in the round, Navrine was reduced to 0hp remaining, she is now Unconscious
19 - Chromir [07/15hp]; Ranged attack, a miss!
17 - Cassian [14/14hp]; Move, ranged attack, a hit! [6hp]
16 - Doc [06/11hp]; Stonecunning active,

Init - Opponent [hp inflicted]; Conditions & Effects; Actions
7 - Inferno Spider [23hp]; Bloodied; Attack Navrine, a hit! [7hp piercing damage]
--> This attack brings Navrine to 0hp remaining.
---> Navrine must make a Constitution Save [DC11] or take 6hp fire damage (no damage on a success)

The Inferno Spider is AC14

Seeing Navrine fall, Cassian dashes over to the spider as well without a second thought. He feels guilty for not rushing over to the spider first when he had the chance, seeing as he is unharmed so far and in heavy armor. At the very least he hopes the creature will target him as the greater threat and leave her alive long enough to be healed.

Action: Dash to Spider

Chromir does not like seeing the spider hurt his new.... friends? acquaintances?, well whatever they might choose to be called, it is still not nice. He lets his anger guide his arm, and throws the javelin with more power than he has before, not caring if it breaks on impact or if it can be reused. 

Attack: 24 (crit) for 11 damage plus slow

Chromir gripped his javelin and hurled it with all his might. The weapon flew through the air and struck the Inferno Spider squarely between its blazing eyes, shattering its carapace in a burst of embers and glowing ichor.

The spider let out a final mournful howl before collapsing, its limbs curling inward and twitching as its flames flickered and died.

Silence fell, broken only by the crackle of burning webbing and the heroes' ragged breaths.

The battle was over.

-End Combat-

Consumables Used
Cassian: One First Level Spell Slot [Divine Smite]
Chromir: Rage (ongoing from last encounter, 2 min remaining), Stone's Endurance
Doc: Stonecunning (ongoing from last encounter, 2 min remaining)

The combat is over, but because Navrine is rolling death saving throws, we will still be tracking things round-by-round until she is either stabilized or dies.

Doc quickly casts a healing spell on Navrine healing her for 8hp. With combat over, and Navrine back up he gets his spear from the spiders body and starts looking around the room for anything of interest (Perception rolled in gamelog 23, if investigation is better suited -5 to that)

"What could have caused this spider to grow that large?"