Torunn eyes off the small spiders crawling everywhere before looking back at the spider dragon, "how about you try this one!" 

He points his left hand and it opens up with crackling blue energy, before shooting a beam of crackling energy.

Lightning Bolt: Dex 17 to take half of 27 lightning damage.

He then follows up with a swing of his right fist that extends on a chain for extra reach.

Attack: 14 Damage: 12

Round 2.4 - Suddenly Passoveloce disappears from the webbed area he is enmeshed in and reappears inside the party’s safe abode - “what the… ?” he says to Lucas who looks at him with a smug self-satisfied grin “never managed to get an opportunity to use that particular spell before!”

Round 2.5 - Balancing precariously on a couple of web strands Corwyn swings his sword at Ker-arach. Once of the swings hit and two just miss but still do some damage. His swinging sword also burns though the webs he was standing on and he falls another 5 feet - 15 {15 rolled and manages to avoid being entangled} but…
Damage: Ker-arach takes a total of 26 points of damage.

Round 2.6 - Toruun traces a sigil on his armour and a bolt of lightning targeting Ker-arach and Xx@^$! blasts out zipping over Corwyn’s head. Ker-arach manages to dodge the blas but still takes 12 points of damage. Xx@^$! also dodges the blast and the disappears from sight  - emerging a further 30 feet away at the far end of the chamber. It takes a total of 3 points of damage.
Damage: Ker-arach takes a total of 12 points of damage, Xx@^$! takes 3 points of damage.

Round 2.7 - Paul clambers and pushes his way through the web to get within striking range of Ker-arach - 22, 11 & 16 {and successfully gets within range of Ker-arach}. When he gets there he swings his great axe - Attack: 17 Damage: 12 and Attack: 21 Damage: 11 plus 3 and 2 Force damage on each hit.
Damage: Ker-arach takes a 28 points of damage and collapses into the web unmoving.

What does the party want to do?

With him still being limited in perspective, Lcuas can only but encourage finishing the fight quickly as to -- if nothing else -- avoid leaving behind witnesses. Though had he thought about it more, that ship would've already sailed given the fact that the others... were very much not disguised when interacting with the other cultist. Or at least most were not, in any event. 

(OOC: Safe to assume we're still in initiative? Or did Xx@^$! die as well?)

Yes you are still in initiative which means its Passoveloce’s turn

Passoveloce moves back into the Webs as far as he needs to to Shadow Step to the rod part. He then proceeds to take possession of the rod, cutting it free if he needs to.

Round 3:1 - Passoveloce leaves the abode and sprints back into the room where he promptly disappears and then reappears in the webs next to Corwyn and the dead Spider Dragon  13. {11 rolled - DC12 required} Unfortunately he becomes stuck when he arrives and struggles like a blowie caught in a Daddy Long Legs web to free himself 6 {5 rolled - DC15 required}. Exhausted he rests then unable to move.

Round 3:2 - Xx@^$! At the far end of the chamber becomes mist like and slips out through a rift in the far side - i.e. away from the side of the chamber adjacent to the rest of the complex!

Corwyn:

Reaching past the dead body of the spider dragon, Corwyn grabs and pockets the rod shard.  Weaving his blazing sword in wide arcs, the Elven warrior attempts to clear the area around the corpse, hopefully freeing Passoveloce and revealing any nearby treasure of note.

(Interact with object to get the rod, 3 sword swings as an action to burn away webs, incidental movement as needed, and passive perception 16 to see if there is other useful treasure or items nearby.)

Corwyn manages to free the piece of the Rod and return to the abode. Almost simultaneously Paul makes his way to Passoveloce and says “c’mon shortstuff - let’s be having you out off that mess!” as he wrenches the halfling out of the webs.

So now what…?

Passoveloce says "Thanks Paul", also muttering under his breath "Bloody webs". Then continues more generally to the group "I think we need to get out of here before the spider alerts everyone or comes back with some friends. But I need to rest for a minute or so."

Once Corwynn and the rest return to the dome with news on what happened, Lucas eyes immediately went wide with concern. "I'm afraid we prolly don't have the time, Passo. At best, they'll organize to block off any escape routes 'fore sending in hunters for us, or even better yet, abandon this place altogether since we've proven capable of taking down the dragon. But at worse? There's enough zealots among their numbers that they'll be bearing down on us at any moment!"

He then looks sharply to Corwyn and holds out a hand. "The rod, if you would. What I might lack in knowledge, I might yet be able to pluck from it a means of a more expedient escape. Either which way, we aughta be braced for the worse over a reckless race outta here." He suggested as much. Yet, if plenty of the others were more in favor of doing just the opposite, for the sake of time the man wouldn't put much of an argument. Regardless, even once handed the wand he takes a moment to weave a spell once again to reinvigorate at least some of the others, before taking a moment to try and quickly study it. 

-1st Action: Casting (5th level)Aid on Corwyn, Paul, & Torunn, increasing Max HP by 20 and healing the same amount up.
(OOC: For those that don't know/remember, if you click on hit points you can adjust your Max HP bonus at the bottom of the pop up, as you /can/ heal up to the new amount thanks to the spell)

-If possible, study the rod to see if it has a plane shift or similar magic available to it. Arcana: 18.  

Corwyn:

Handing over the rod, Corwyn murmurs his thanks for the fortifying spell and awaits Lucas’ findings.

(OOC: Actually dm, if studying it would take the same amount of time as casting identify, Lucas will instead 'try' to cast it on the rod using his Fey Touched feat. A likely bad idea/doomed idea given its an artificat, but oi, gotta save some time some how)

By studying the Rod and casting Identify Lucas is able to work out that he can cast Commune. He is also able to work out that he can’t attune to the Rod until it is complete.  

"Aww, fudge monkeys! The rod's a bust, boys. Looks like we're gonna need to figure out our own way out at this point. Which brings us back to that point of order. The dome aughta protect us to some degree if the rest come snooping, if resting for a bit is the play. But I'll warn ya that powerful enough magic can yet bypass it. And considering humpty dumpty back there managed to summon a demon as powerful as the one we saw, odds are he at least got it to spare." Lucas explained as quickly and concisely, and with rising panic just barely held back out of his voice. "So, what'll be. We taking our chances or we holding up in here and hoping fer the best? My vote is chancing it, but we gotta decide quick."

Corwyn:

“Let’s head back to the entrance like nothing is wrong.  Hopefully it will take some time for the spider to convey the alarm,” suggests Corwyn, though he considers the layout of the complex for an alternate route to the exit that doesn’t go by the large group they passed on the way in.

"Well, we can take off now, but when this haste wears off, I'll have a turn when I will be as useless as a halfling trapped in a web." says Passoveloce, With a downcast, and disappointed demeanour. "But we do not have time to waste. I think we need to get past this summoner and his demon before the alarm is raised. So let's get going."