Shipwrecked in Saltmarsh

The captain sputters with anger, unable to respond, or even to process much of what you're saying. "GET IT OFF MY SHIP!"  He storms off, returning up to the outer deck.

(Kelnan - are you suggesting you want to do this now? Do you have any specific preparations you need to make? Does anyone have anything they want to prepare Kelnan with, first? Do you have any specific instructions on how you want to launch that encounter?)

     “But… it’s not on your ship.  You can tell that by the lack of dwarven death screams…” Gianba says quietly after the captain storms out.

Molo would inspire Kelnan before his venture

”Kill the spider. And not us.” (Inspiring! 1d8 to any attack roll, ability check, saving throw, AC or damage of a weapon attack)

Kelnan quiets and prepares himself mentally for his dangerous attempt to fix this situation. 
With some fussing and gesticulations, he prepares the first spell. Flames begin to flicker off the fingers of one hand.
He tries to maintain a non-confrontational stance, so as not to alert his foe. Stealth: 5
Then he mutters a few more arcane words and makes a gesture with his other hand in a seemingly impossible direction. Blink: 2

Six long seconds pass; nothing happens. A bead of sweat runs down Kelnans forehead. The flames continue to flicker on his clenched fist.

 Blink: 17

With a tiny *pop*, Kelnan vanishes into a direction you didn't know existed.
There is silence on board as everyone stares at the empty space where Kelnan stood.

All is quiet.

(Kelnan: Watch for PMs.)

A few seconds pass. An unnaturally warm breeze blows through part of the cabin.

Then Kelnan reappears. He is uninjured, but slightly out of breath and has an alarmed look on his face.

Kelnan

"Damned thing is quick on his home ground. Keep your wits about, in case it emerges."

He readies a Ray of Frost, and waits patiently.

Attack whenever it happens: 24 Damage: 8

     Fianna blinks when Kelnan disappears but readies a spell herself.  When he reappears she has a spell ready, but not readied, in case the spider should show.  (Basically just ready like any good adventurer would be.)

     "What happened?  So it's not dead?" Fianna asks, hurriedly and looks about to see if the eight legged thing is going to follow...

Molo starts to prepare Shatter magic, then thinks better of it. He’s now ready to Bane the creature if it appears. 

Let's start with initiative:
Kelnan: 14
Spider: 12
Fianna: 14
Katernin: 10
Molo: 14
Dain: 3
Dwarven crew: 14

The spider has not appeared.
Kelnan readies another spell.
     Blink: 18
He vanishes again.

With the spider and Kelnan still absent, the rest of you continue to prepare.
Molo readies a spell.
Fianna prepares herself for a fight.
Does anyone have any further actions?

     Thinking more on it, Gianna would ready and hold Faerie Fire to cast upon the spider in reaction to his appearance…

     And the next time Kelnan reappears, Fi’Finella is going to invisibly land on his shoulder and say “Take me with you…”  The Imp can’t do much but will try to fly off Kelnan’s shoulder if brought to Ethereal Realm to kind of watch the spider and see if it can get a sense of how it’s keeping pace with the ship…

(So not sure it will work but Imp familiar trying to hitchhike a ride to other realm by being on Kelnan next time they blink and will then try to stay there by flying off him…)

(If Kelnan has Bardic Inspiration active again, he would certainly use it to bolster a low roll, not sure how often that happens though)

Dain has hefted his maul and is looking really confused.  If a big blue ugly thing materialises within reach he will try and squish it.  Not much else he can do, I don't think!

((Ready an Attack on seeing the spider appear within his threatened area))

And the next time Kelnan reappears, Fi’Finella is going to invisibly land on his shoulder and say “Take me with you…”  The Imp can’t do much but will try to fly off Kelnan’s shoulder if brought to Ethereal Realm to kind of watch the spider and see if it can get a sense of how it’s keeping pace with the ship… (So not sure it will work but Imp familiar trying to hitchhike a ride to other realm by being on Kelnan next time they blink and will then try to stay there by flying off him…)

Blinking doesn't normally include hitchhikers, but Imps are strange creatures, and RAF/RoC may apply here once again. Make a DC15 arcana check on Fi'Finella's behalf for the next successful blink (and for each thereafter) to see if it can successfully hitchhike.

Kelnan is pretty sure plane shifting unrestrained arcane entities violates some kind of quarantine rules, but who would ever find out? The border ethereal barely counts as a separate plane anyway, plus that's some elvish nonsense in the first place, his human colleagues would probably celebrate it. "Hang on then, any second now..."

Arcana 6 for Fi’Finella 

Second attempt, if needed 16

After two attempts Fi’Finella and Fianna would give up.  It was a long shot anyway…

((Definitely iffy rules wise and totally understand if doesn’t work.  Likely won’t discover anything if it does anyway…))

Kelnan reappears again. Again he looks alarmed and perhaps a bit unnerved but not injured. Fi'Finella lands invisibly upon his shoulder.

Still no spider appears.

(Is Kelnan preparing another spell? Let me know which!)

*Blink* 7

 Nothing happens. Kelnan stands on edge, Fianna's invisible imp continues to perch upon him. 

That's when the spider appears, massive, seriously burned, but still unnaturally blue. The floorboards creak under its weight.

It springs upon Kelnan.

Bite Attack: 23  Hit: 9 piercing damage (and on a hit Kelnan must make a DC 11 Constitution saving throw, taking 15 poison damage on a failed save, or half as much damage on a success.)

 The beast fails to penetrate Kelnan's arcane armor with its poisonous fangs.

A cascade of readied attacks fall upon the party's strange attacker.

Dain: Maul attack: 17. Hit: 8 bludgeoning damage

Fianna releases a strange blue glow upon it: DC 15 Dex Save: 3...

...while Molo casts a curse upon it: DC15 CHA Save: 2

Katernin and Kelnan may act next against the successfully baned creature.