Shipwrecked in Saltmarsh

Molo, covered with his own swarm, manages to scrape a few off of Dain. But there are a lot left. (Damage resistance to slashing and piercing.)

Fianna, Kelnan, Dain, and Katernin may act (in any order.) The wounded hairy spider has vanished; all that is left are the two swarms covering Dain (more) and Molo (fewer).

     For the moment Fianna has little choice but to try to Eldritch Blast the swarm off one of them.  As Molo is the smaller of the two, she tries to help him first…

Attack: 6 Damage: 9

Fianna fires a blast of crackling energy at Molo's infestation. It goes poorly. Instead of disposing of the spiders, it shears off most of his pant leg, exposing his koboldy thigh. One leg of his trousers is hanging on by only a thread now.

Kelnan, Dain, and Katernin may act next.

((Wasn't me!  It was the moths! I  seen 'em!  I swear!!))

Dain is going to try and attack the swarm with his maul, from his position lying on the floor covered with webbing!

OOC:  I have Disadvantage for being restrained, Disadvantage for being prone.  However I believe that if I use Reckless Attack which gives me Advantage to attack, they all cancel each other out, regardless that I have multiple cases for Disadvantage and only one for Advantage.  So I'll make my attack normally and hope I am correct.  Dain is still raging, which adds to his damage.  Also, Reckless grants Advantage on attacks against me but I'm already granting that for being prone and for being restrained so it's insignificant.

Attack: 6 Damage: 18   against his own swarm.
Note:  Granting Advantage to all attacks until my next turn.

Kelnan fires another Ray of Frost, hoping Molo is still okay in there.

Attack: 17 Damage: 6

From his awkward position on the cabin deck, Dain manages to free his maul. He attempts to mash the spiders crawling up and down his body, but all he succeeds in doing is cracking his kneecap painfully and getting his maul stuck in the webs. (Thanks for talking through the adv/disadv and the damage - it's very helpful, and I agree with your rule interpretation.)

Kelnan has better luck with Molo's spiders with his icy beam, nearly destroying all of them. There are just a handful of toothy beasts infesting the tough, one-pantlegged kobold.

The little biters continue to feast on our heroes:

Swarm #1: Reduced Bite Attack vs. Molo: 13  Piercing damage: 6
Swarm #2: Bite Attack vs. Dain: 10  Piercing damage: 15

Fortunately, they manage to fend them off, for now.
Everyone may act, starting with Katernin and Molo.

Molo keeps swiping at Dain’s spiders

Attack: 15 Damage: 5 

Molo manages to slash a few more off the restrained dwarf, but there are a lot more ready to feast on him.

Everyone but Molo may act next.

     If Fianna is able to get within 10’ of both swarms she is going to try to use her Fey Presence to try and make the swarms frightened of her and to flee…

Fianna's eyes flare, and her aura pulses with unworldly menace. Her presence swells with ominous terror.

But the spiders continue their feast, undeterred. (Condition Immunity: Frightened)

Kelnan, Dain, and Katernin may act next.

"I've had enough of this!" Dain roars in frustration and swings his maul at the swarm covering him again (same approach as last round).

Attack: 12 Damage: 12

OOC: In the unlikely event that is enough to destroy the swarm on him he'll use his bonus action (due to GWM) to swing at the swarm on Molo, if he can reach when restrained.

Bonus Attack: 25 Damage: 17

Quote from cbaer8 >>

But the spiders continue their feast, undeterred. (Condition Immunity: Frightened)

 ((Demmit! That’s what I get for not metagaming!)) :)

Even from his awkward position restrained on the floor, Dain furiously mashes the bugs crawling all over him.

This time he does much better, crushing the majority of them. But not quite all - there are still a few left. (No attack on Molo's, alas.)

Kelnan and Katernin may act next!

Kelnan

Wiping splattered spider guts off of his waistcoat, Kelnan takes aim at the remaining bugs on Molo.

Attack: 14 Damage: 3

With his icy white beam, Kelnan narrowly misses searing the few remaining arachnids crawling on Molo.

Katernin may act next!

Katernin lifts up her shield at a point between the two, and a large quantity of water appears. It drops on them both, trying to wash the spiders away.

Casting Create or Destroy Water

Water pours over Katernin's two companions - a one-pantsed kobold and a furious, webbed, prone dwarf.

(We'll make it a DC14 strength check for the spiders to hang on.)
Molo's spiders: 0
Dain's spiders: -2

The toothy little spiders are washed away. The water floods the cabin and then drains through the web-covered, wooden grate on the floor in the middle of the cabin.

The retreated, wounded spider is nowhere to be seen. Combat is over (at least for the moment!)