DM rolls:

Spider Spat! - Round #1 - Initiative

An eerie blue glow covers the hallway, radiating outward from the spiders.  The giant phase spider stands on the crumbled bodies of the two slain tree knights, its hefty, bloated body blocking the way through to the bedroom.  Hundreds of tiny spiders crawl around its massive legs, and their little chelicerae ((learnt a new word!)) twitch as they advance on the party.

Tree Knights Turn

Tree Knight Captain

Trying not to dwell on the carnage already wrought by the spiders, the Tree Knight Captain raises its right hand above its head.  As before, numerous small roots begin to grow and wrap around the giant phase spider, attempting to lash it to the ground.

• Root Grasp: Attack: 21, Damage: 9 (slashing)

Though the roots fail to take hold of a creature the size of the phase spider, they nonetheless cut into its legs and abdomen.  The Captain proceeds to lurch forward and attempt to stab at the spider with its sword.

• Thorn Sword: Attack: 10 (miss)

Unfortunately, the sword harmlessly scrapes along the spider's thick legs.

Tree Knight #1

Supernaturally aware of the blight of its companions, Tree Knight #1 storms forward toward the nearest swarm of spiders.  Using its thorn sword, it swings down at the spiders, attempting to kill as many of the tiny creatures as possible.

• Thorn Sword: Attack: 20, Damage: 3

Though it manages to hit several of the spiders, the swarm does not look very diminished in size.

Tree Knight #2

Joining in the fight, Tree Knight #2 strides up to the other swarm of spiders.  It attempts to crush as many of the creatures as possible with its shield.

• Thorn Shield: Attack: 9

Unfortunately, the vast bulk of the swarm seems unaffected by the Tree Knight's attack.

Ermasnietsz's Turn

Ermasnietsz curses at how quickly events had escalated.  Taking out her crossbow, she looses a bolt at the giant phase spider.

• Crossbow, light:  Attack: 20, Damage: 8 (piercing)

Her aim is true as the bolt whizzes between the Tree Knight Captain and Nikita and thunks into the giant spider, protruding out of its sternum.  Pale blue blood seeps from its wound.

Nikita - You watch as the spiders flicker in and out of existence.  Quickly coming to the realisation that every element, including their innards, have been dyed a peculiar blue colour.  It is clear that this discolouration is unnatural and most probably some form of arcane taint.  Additionally, you note that the spiders appear to be acting with increased aggression.  Currently, the larger spider appears to be feeling the effects of the Tree Knight Captain and Ermasnietsz attacks, it already seems a little unsteady on its feet as blue liquid continues to stream from its wounds.  It is your turn.

Battlemap:

((Blue box denotes size of Giant Phase Spider))

Initiative Order:

1. Tree Knights (2/2) + Captain - 22 - Green Tree Symbol & Small Green Shield Symbols
2. Ermasnietsz - 21 - Orange Infinity Symbol
3. Nikita - 18 - Purple Wand Symbol
4. Boyd - 17 - Tan Dagger Symbol
5. Spider Swarms (2/2) - 17 - Small Pale Blue Shield Symbols
6. Giant Phase Spider - 15 - Pale Blue Spider Symbol (creature is a large monstrosity and occupies a 10ft x 10ft sqauare)
7. Meresaa - 11 - Dark Blue Gun Symbol
8. Faila - 3 - Pink Staff Symbol

Humming her quiet dirge, her feet made light by her bladesong, Nikita stows her rapier and at a gesture, a razor-thin blade of gloomy shadow coalesces in her hand, seeming to drink the blue light from the spiders.  Bonus Action:  Shadow Blade

As Thumbtack flies by the giant blue spider to provide help, she slices at the monstrosity with her Shadow Blade using her Green Flame Blade cantrip to make eerie fire arc from it.  However, the unfamiliar feel of the weapon throws off her balance and she misses what should be an easy strike.

Shadow Blade to hit:  10
Damage on hit:  2d8+5 = 13 psychic
On hit, Green Flame Blade causes fire to arc to the northern spider swarm (#1) for an additional 4 fire damage.

"Blimey!" Boyd exclaims in a panic. "It's huge!" As it's struck by the crossbow bolt, he takes the opportunity to tumble backward (bonus action disengage) and head half way up the stairs. From the high ground, he lobs the returning knife at the bright blue eyes of the giant spider.

Attack: 7...

Spider Spat! - Round #1 - Nikita & Boyd's Turns

Nikita's Turn

Nikita is quite the sight as she dances forward, flaming shadow blade in hand, she is the epitome of grace and agility.  That is, unfortunately, until it comes to striking the phase spider.  Just before she is able to land a near-lethal strike on the creature, it momentarily phases and Nikita is caught off-guard, her blade finding only air.

Thumbtack, meanwhile, shrieks and swoops near the phase spider, distracting it.

Boyd's Turn

The stairs floorboards creak and groan as Boyd takes his time to rush up them.  Many of the stairs are rotten and look like they may give way at a moment's notice.  Throwing his knife at the spider, it fails to strike the creature's eyes and instead harmlessly bounces off its thick hide.  From his vantage point, he can only watch helplessly as the swarming spiders attempt to lie low the remaining Tree Knights.

Spider Swarms' Turns

#1. The swarm of spiders to the north scuttle over the closest Tree Knight (#1).  Their tiny blue mouths attempt to find any openings or holes within the Knight's wooden limbs.

• Bite:  Attack: 18, Damage: 14 (piercing) ((close to max damage!))

A gruesome sight follows, as the Knight swats and bats at the tiny spiders to no avail.  They swarm over the once-noble Knight, their tiny bodies flickering as they continue to phase in and out of Prospect.  The Knight's movements grow increasingly more panicked as more and more of the spiders cover its wooden body and ancient armour.  The Captain holds out a hand towards its companion as it slumps against the nearby wall, struggling to hold its own weight.

#2.  The southernmost swarm rushes over the other Tree Knight (#2).

• Bite:  Attack: 16, Damage: 11 (piercing)

Similarly to the Tree Knight to the north, the southern Tree Knight frantically attempts to beat away the encroaching spiders.  The hundreds of tiny glowing spiders light the Knight up as they swarm over it; they move in and out of the Knight's armour and wooden frame.  Attempting to maintain its composure, the Knight ineffectually swings its thorn sword, failing to make any serious contact with its foe, before toppling over backwards.

Neither of the Knights has perished, though both appear overwhelmed.

Giant Phase Spider's Turn

Distracted by the fate of its soldiers, the Tree Knight Captain barely notices the giant spider's maw before it comes biting down.

• Bite:  Attack: 19, Damage: 10 (piercing)
• Tree Knight Captain Constitution Saving Throw: 4 ((fail))
• Additional Damage: 13 (poison)

The spider's fangs wrap around the Tree Knight Captain, denting and piercing its armour.  Though the Captain's grip on its sword seems to loosen a little, it regains its composure and attempts to detach itself from the spider's mouth.

Meresaa's Turn ((Botted))

Watching in terror as her new-found allies become overwhelmed, Meresaa mutters an incantation under her breath and stretches out her right hand.  Flames lick around her hand before coalescing into a fiery mote, which she proceeds to launch at the giant spider.

• [Tooltip Not Found]:  Attack: 22, Damage: 5 (fire)

The [Tooltip Not Found] streaks through the air and crashes into the spider, searing a portion of its skin, and turning it a darker shade of blue.

Faila - Meresaa turns to you, a look of desperation on her face.  "Faila," she cries, "we have got to do something!  They are our allies!"  The Tree Knight Captain continues to struggle against the giant phase spider and the two other knights' fates are hanging by a thread.  Desperate times call for desperate measures, it is your turn to act.

Battlemap:

Faila looks desperately at Meresaa, and then at the tree knight captain with the giant spider sinking its fangs into the captain's creaking wood carapace. The spider! It's a fae spider, but it's still a spider! It's just hungry! She reaches out a hand towards it, reaching for the magic of nature around her, trying to see if the spider still fits in, if she can make an Animal Friendship with it.

Spider Spat! - Round #1 - Faila's Turn

Faila's Turn

((The giant spider theoretically is classed as a monstrosity rather than a beast.  However, overlooking this, it also has, unfortunately, an intelligence of more than 4.  That said, rather than the spell simply failing the following happens...))

• Phase Spider Insight: 12

The phase spider stares curiously at Faila, cocking its head to the side.  Though the creature does not appear to be under the effects of her spell, it nonetheless appears to understand that Faila is attempting to communicate with it.  The giant spider defensively crams the bulk of its body into the door frame, clearly seeking to block the passage.  Several of the smaller spiders scuttle back to the larger spider and proceed to clamber over its huge body.

You surmise that the spiders are more concerned with preventing access to the room rather than sating their hunger.  Perhaps making clear that the party is not aiming to enter the space might stop the creatures' attacks.

Faila watches the spiders as they defensively cram into the doorframe. They're not... well, they're not. But I think I understand what they're trying to communicate. They are smarter than I thought. She speaks up to her companions. "Friends, the spiders do not want us to come into the room! Perhaps if we stay out of the room, they will not attack us. We might not need to fight them. They are intelligent, for spiders."

Spider Spat! - Round #2 - Tree Knight's Turn

Tree Knight's Turn

• Rolling to check the Tree Knight Captain's reaction: 18

The Tree Knight Captain's wooden head snaps around to look at Faila.  Instantly comprehending the message she is conveying, it waves its hands, beckoning its knights back.  Both of the remaining knights move forward, grab their downed companions, and proceed to drag them back away from the room.  Strangely enough, the spiders do not impede their efforts or attack as they disengage.

Ensuring that its knights are safe, the Captain readies an attack in case the giant spider attacks.

Ermasnietsz's Turn

Ermasnietsz scowls at Faila.  "And what if the spiders guard a powerful artefact, Faila?" she asks sardonically.  "We're just to pass up on this opportunity and for what?  To save the lives of a few creepy crawlies?"

Nonetheless, similar to the Tree Knight Captain, she readies an attack on the giant spider, knocking her crossbow, just in case.

Nikita - You watch as the Tree Knights retrieve the remains of their fallen.  Both the Captain and Ermasnietsz have attacks readied in case the spider attacks.  However, it appears as though a peaceful resolution is being reached.  It now falls to you to decide how you proceed; do you follow Faila's lead or continue to pursue the items contained within Asha's room?  The decision is yours, it is your turn.

Battlemap:

Initiative Order:

1. Tree Knights (2/2) + Captain - 22 - Green Tree Symbol & Small Green Shield Symbols
2. Ermasnietsz - 21 - Orange Infinity Symbol
3. Nikita - 18 - Purple Wand Symbol
4. Boyd - 17 - Tan Dagger Symbol
5. Spider Swarms (2/2) - 17 - Small Pale Blue Shield Symbols
6. Giant Phase Spider - 15 - Pale Blue Spider Symbol (the creature is a large monstrosity and occupies a 10ft x 10ft square)
7. Meresaa - 11 - Dark Blue Gun Symbol
8. Faila - 3 - Pink Staff Symbol

To say Nikita is doubtful about the unfolding negotiation and truce with the otherworldly spiders is an understatement.

But in her head, a voice counters:  If you do not trust this wild young woman on one of the very few occasions that she has taken the lead to guide the group, then when will you ever trust her?  When this is all over and you and she are corpses?

Nikita maintains her position, her bladesong and her Shadow Blade while Thumbtack flies in a circle above her head.  Unlike the Tree Knight Captain, she does not ready an attack but instead dodges in case the spiders can't help themselves and attack anyway.

"Asha Hawkes.  We must know now before we either disengage or press the fight to gain access to your room.  Your mandolin.  Your candles.  Your starglass.  Do we need them?  Do you need them?  What can one do with them and might they help us secure the portal or free you?  And who were you saying earlier was coming and would show us no mercy - these spiders here or something or someone else?"

Nikita - A breeze blows through the manor, chilling you to the bone, on it Asha's voice is carried.  "Nikita, I shall attempt to discern what I may, though I find myself becoming increasingly overcome by lassitude each time I try to look into the room.  It is almost as though something else dwells within, a force perhaps," she says, meekly.

"I see... blue.  A sea of blue light.  It covers a blue bed, with blue curtains, and my mandolin!  It is as though all other colour has been drained from the world.  I do not know what is causing it but urge caution, Nikita.  The artefacts can wait.  Of all the items, the starglass is the most useful for it would act as a window to and from the Feyrealm.  However, these items are a luxury."

There is a pause before Asha's voice comes through weaker still.  "I misjudged the spiders.  It was they that I was referring to.  So eagerly did they appear to defend whatever force has changed their colour.  I did not know they could be reasoned with," her last words come through as almost a whisper.  "I must rest."

"Thank you," Nikita whispers back to the ghostly voice of the lost woman.  "Rest, and do not weaken yourself further.  Same goes for us."

"My sense is that we trust Asha and fall back.  The items inside the room may not be worth fighting the spiders and whatever blue force lies beyond.  Disengage carefully and do not turn your back to them until we are well away.  You were right to talk to them, Faila."

Boyd has been watching the arachnid negotiation unfold with skepticism. After all is said and done, he shakes his head as though, even after all he's seen in the last couple of days, this is too much to believe.

"Well we've gotta put a sign on the door for Nick or what not - 'Do not open. Smart blue giant spider inside.'" He says while backing away up the stairs. He looks at the fallen wood elves. "And what about them? They're not looking so good."

Nikita smiles wanly at Boyd's observations.  "Aye.  Can't disagree - no good solutions.  Nothing new.  Just have to make the best of it."

She signals and waits for the others to retreat, continuing to dodge.  Still bladesinging with the Shadow Blade in her hand.

With her free action, Nikita hands a Potion of Healing to the Tree Knight Captain, addressing him in Elvish:  "I know not if your soldiers can benefit from such, but if they can, this is the least I can do.  Drag them to safety and help them."

She continues in Common:  "Faila, if you can help revive the tree-elf warriors once we are safely away, please try.  I will be the last to fall back.  After that, we should all rest the night before continuing.  This house is too dangerous to explore half-ready."

Night - Day 3 - The Better Part Of Valour

Crawling over themselves, the swarms of blue spiders retreat back into Asha's bedroom.  The giant spider barely manages to squeeze its large abdomen through the door before slamming it shut, leaving the party to contemplate the bizarre interaction.

The Tree Knights manage to drag their downed comrades from the entrance to the room unimpeded.  Though most of their wooden frames have been gnawed and broken, it looks as though fresh life is beginning to sprout from the dead wood.  Eagerly, the Captain takes the potion from Nikita and applies it like a balm to its damaged knights; the dead wood cracks and splits as it is replaced with living, similarly to a snake shedding its skin.  Previously inanimate, the downed knights spring back to life, pulling themselves to their feet.

Webs cling to the outside, as well as presumably the inside, of the door to Asha's bedroom, serving as a stark warning to future visitors.  Sparing no time, the Captain moves to venture upstairs before hesitating a moment and waiting for the party's command.

Last to disengage, Nikita finally retreats, holding her bladesong and Shadow Blade until the last.  The scar on her cheek stands out in strange, livid relief, bathed in the fading unearthly blue light of the spiders in the distance.  She looks suddenly very tired and very young.

Seeing the Tree Knight Captain's eagerness to continue, she tries to gauge the extended party's consensus in both Common and Elvish:

"We have to decide whether to continue today.  Many of our resources are exhausted - well, at least mine are.  I no longer have the energy for another bladesong in me without the cleansing of a trance, and some of you need a longer rest than we've taken since just after rescuing Esther.  I vote for retreating to a known, relatively safe (?) spot, perhaps the Observatory or hidden Old Aerdy treasure trove, and setting up watches for the night.  But if some of you feel the urgency of pressing on, with the threats here unknown and the portal deteriorating, I can accept that too."

Boyd watches as the wood elves come up the stairs with him, and listens to the point made by Nikita. "Hmmm. I suppose I'm alright, but if our magic folks need a lie down, I'd rather stay in the old treasure room. That observatory don't sit right with me after what we saw..." He shudders, recalling the blobby Sedgewick.

Nikita looks around and nods.  It is difficult organizing so many to work together.  Is this what it was like for the leaders of my coterie?

"Is everyone agreeable to retreating to the Old Aerdy secret treasure room for the night?  Or does anyone feel we should press on now?  Perhaps due to the urgency of the deteriorating portal?"  Nikita repeats this in Common and Elvish as needed (asking as yes-no questions for the tree knights).

To Boyd:  "You're right.  It was bad enough we had to take a brief rest in the spot where the Sedgewick abomination died."

"I'll leave portal business to you folk." He says heading toward the treasure room secret entrance. He turns to the wood elf captain on the walk there. "Your boys okay after that healing potion what Nikita gave you?" He spares a glimpse for the new wood that sprouted up to replace the spider-torn old wood on the two soldiers. "So, you're them wood elves I've heard about, eh? You been here a long time by the sounds of it... You uh... like it here? In Oerth, I mean?"

Hairless brow knitting, Meresaa takes a deep breath and mentally walks back through the last several moments: a confusing series of events seeming to have stretched on for an eternity.

"I'm well, but it would behoove us to regroup and tend to our various needs," she states. "This has been quite a day and it seems to me tomorrow could prove to be even more so. Let's retire to the relative safety of a hidden room and lick our wounds. There will be time to revisit Lady Asha's room if that's determined to be a desirable path forward, but not in our current state."

She gives a faint smile and nod to Nikita, indicating her approval of the seemingly young elf's leadership thus far, and lays a reassuring, long-fingered hand on Faila's shoulder as if to say 'Well done.'

"Besides, I've been meaning to spend some time with those notes..."

Investigation or Arcana (same mod, with advantage from help): 18 (+3 if arcana, from Tireless Precision) to deciphering the documents and equations during the long rest as she's been studying the seemingly unpredictability of planar rifts prior to this adventure.