Shipwrecked in Saltmarsh

Katernin snaps her fingers, and tries to burn away the spiders on Molo.

Action: cast Sacred Flame: DC 14 dex save or take 7 radiant damage

Katernin pulls down a holy radiance from above, engulfing Molo's little attackers. The scramble to avoid the light - Dex save: 3

But they cannot avoid it. Their numbers are significantly diminished, but not yet eliminated.

Meanwhile, Fianna fires a beam through the doorway, narrowly missing the big spider, but only succeeds in burning more webbing. 

 Kelnan may act next.

Kelnan continues to blast away at the spider. Surely something will land eventually!

Attack: 8 Damage: 1

Kelnan scores a direct hit with his blue-white beam of ice, causing a serious wound, and two of its hairy legs to freeze together.

The injured, frost-covered spider, however, continues its attack; this time, on Katernin:

Spider Bite Attack vs. Katernin: 18  Piercing damage: 2 and on a hit she must make a DC 11 Con save or take 6 poison damage; or half as much damage on a success.

It cannot get through Katernin's thick mail.

Dain is up next!

Dain and Katernin may both act. (Both still may use inspiration, I believe?)

Dain and Molo, near the hatchway, are covered in swarms of biting spiders. One big hairy spider, wounded, continues to attack from beside the door; the other has retreated into the webby shadows. The bipedal one has also retreated somewhat, but remains visible to all.

Initiative (modified slightly just now for PBP convenience):
Fianna     √
Kelnan     √
Dain         may act next
Katernin  may act next
Big hairy spiders
Swarms
Bipedal spider
Molo 

Dain ignores the swarm of biting creatures swarming all over him and tries to crush the huge spider he can see and reach.

Reckless Power Attack: 18 Damage: 30
Note:  Granting Advantage to attacks.

If that is enough to kill the spider he will step forward to where he can try and reach the bipedal creature and use his bonus action to attack it (due to GWM feat):

Reckless Power Attack: 11 Damage: 23

Covered with biting pests, Dain destroys the big spider with far more force than necessary, spraying foul-smelling spidery bits across the cabin. 

Stepping forward through the doorway, at the edge of the big trapdoor, Dain then administers nearly as powerful a blow to the bipedal bug, nearly killing it as well. It is now barely hanging on to life.

Katernin may act next. She has a ranged shot at Dain's bipedal foe, the big, wounded, hiding spider, or the swarms infesting both Molo and Dain.

The swarms feast on their hosts:

Reduced swarm Bite Attack vs. Molo: 12 Piercing damage: 6
Swarm Bite Attack vs. Dain: 23 Piercing damage: 8

The bipedal insect, at death's door, tries to spray Dain with webbing from its spinneret. (Close range disadvantage cancelled by advantage.)
Ranged Web Attack: 19  Hit: Dain is restrained by webbing. (As an action, he can make a DC 11 Strength check, escaping from the webbing on a success.)

The tine spiders have completely infested Dain, inside his clothes and out. Their teeth are deadly sharp. Crit damage: 10

Everyone may act, starting with Katernin and Molo.

OOC: Even with rage that is enough to cause Dain to collapse unconscious.

Molo attempts to heal Dain with cure wounds for 6 

Molo, covered with his own swarm of biting arachnids, steps forward through the doorway and revives the fallen dwarf. 

Katernin, Fianna, Kelnan, and Dain may act next. (Dain is prone and restrained by webbing.)

     "Sorry.  Pardon me.  Excuse me," Fianna rattles off, moving past Katernin and Molo in order to get to Dain.  Once there she utters a short invocation and taps him on the forehead (( Cure Wounds 2nd level)) curing him for 12 hp.  She will also do whatever she can to assist Dain with getting out of the webbings...

Kelnan does his best to freeze the spiders off of Molo with Ray of Frost

Attack: 24 Damage: 4

Fianna pushes through and out the doorway to aid Dain. 

Kelnan, meanwhile, nearly hits her with an errant blast of his icy magic.

Katernin and Dain may act next.

The party continues their battle in the dark, listing, web-filled cabin.

Dain and Molo are covered with biting swarms of vicious little spiders. Dain is prone and restrained by webbing. One wounded, hexed spider has retreated into the shadows where only Katernin and Kelnan can see it. The bipedal insectoid is badly wounded. Molo's swarm is reduced in numbers. 

Katernin and Dain may act next:

Katernin, still concentrating on the Shield of Faith protecting Dain, has a clear ranged shot at the spider and the bipedal insectoid, as well as the two swarms on Molo and Dain. She also has inspiration from Molo.

Dain is prone and restrained. As an action, he can make a DC 11 Strength check to escape the webbing. He also has d6 inspiration from Molo which he can use. (But he doesn't have advantage on the roll unless he rages again.)

Dain screams in anger and tries to break out of the webbing.

Bonus Action:  Rage
Action: Strength check with Advantage due to rage:  6

Having failed to break out of the webbing Dain proceeds to thrash around helplessly.

Dain struggles with anger at his sticky bonds, but only manages to make himself more stuck. The thick grey strands, tough as leather, adhere to his hair and bind his limbs. (He can still swing his weapon from his awkward position on the floor, but only at disadvantage.)

Katernin may act next.

Katernin raises up her hand and sends a radiant bolt at the insectoid.

Action: Cast Guiding Bolt (2nd Level): Attack: 15 Damage: 17
+ Inspiration: 3 
a roll to force other rolls to show: 4

Katernin fires a bolt of potent radiant energy at the insectoid. A potentially sloppy, dicey shot, Molo's words give her extra focus. She flattens the bipedal beast. It falls backward, quite dead.

The hiding spider retreats further into the back of the gloomy, web-filled cabin. It vanishes from view for all.

The swarms, however, continue to feast upon their hosts.
Swarm #1: Bite Attack vs. Molo: 13  Piercing damage: 3
Swarm #2: Bite Attack vs. Dain: 11  Piercing damage: 14

Everyone may act next, with priority to Molo. Dain is still prone and restrained by the webbing.

Molo swipes at Dain’s spiders with his scimitar

Attack: 17 Damage: 5